When I was at the beach, I accidentally spilled someone’s drink. At the beach a cup costs four dollars but at the local supermarket I can buy a whole bottle for two dollars so a cup there costs about fifty cents. Do I owe four dollars or can I just pay fifty cents since that is all it really is worth and the stands at the beach just take advantage of the fact that people want to drink soda and there is no other source at the beach. Furthermore, if you rule that I have to pay what it costs at the beach, does it make a difference if he brought the soda from his house and he himself only paid fifty cents?
There is a section of Gemoro (BM 99B) that sheds much light on your question. According to the authoritative explanation of the Rosh (BM 8, 17) (his opinion is ruled in CM 304, 5), the Gemoro discusses a porter who broke a barrel of wine that belonged to a store that had two prices. On market days the store sold a barrel for five coins but on non-market days it sold it for only four coins. The porter broke the barrel on a market day. The Gemoro ruled that the porter owes five coins since that was the amount of money that the store could have received on the day that the porter broke the barrel. However, if the porter returns a barrel on a market day and the store can sell the wine on that day, the porter does not need to pay five coins since the store can immediately get five coins for the barrel. The Rosh adds that the porter is not allowed to delay payment until the market day in order to pay with a barrel since the store owner can demand immediate compensation for the loss that he suffered.
From this case we can derive various basic principles. First, we see, as we wrote in a previous article, that when one damages an article that belongs to a store he is required to pay the price that the store charges and not the price that the store owner paid. We see this since in the case of the Gemoro obviously the store owner spent even less than four coins and yet the porter was required to pay five coins or a replacement barrel. Second, we see that one cannot delay paying for damages in order to save money in case there is any reason the victim wants to be paid immediately. Third, we see that the price is determined at the time that the damage was done even if it is certain that the price is only temporary.
In your case the question is slightly different because the disparity between prices is not based on time but on place.
However, we see in a few places in the Gemoro that price is determined by the exact location. The Gemoro (BK 11A) discusses an ox that fell into a pit which someone dug in the public property and died. The one who dug the pit is liable for the difference in value between a live animal and a dead carcass. The Gemoro rules that if the price of the carcass at the bottom of the pit is one zuz and on the ground next to the pit it is four zuz and the animal was worth fifty zuz when it was alive, the amount the owner of the pit has to pay depends on whether he takes the carcass out of the pit or not. If he removes it and then returns the carcass he owes forty-six zuz but if he returns it when it is still in the pit he will need to pay forty-nine zuz.
Rav Shimeon Shkop (BK 13) explains that the owner really owes fifty since that is what he damaged. The reason his price is reduced by the value of the carcass is because the owner of the pit has the right to repay his debt with the carcass. If he gives the owner of the ox the carcass at the bottom of the pit he is considered as having paid only one zuz, whereas if he pays with the same carcass on the ground immediately adjacent to the pit, it is considered as if he returned four zuz. Thus, we see that the amount that he is considered as having paid is the value at the exact place where he paid.
Thus we see that if a cup of drink sells for four dollars at the beach, the fact that in town – which may be only five minutes away – the price is only fifty cents has no bearing on the value. If it sells for four dollars at the beach that is its value and that is what you owe him if you spill it.
Furthermore, we see from the first Gemoro that you can’t tell your victim that you will pay him the next day at home since the victim can say that he wants to drink now, just like the store owner can demand immediate payment because he needs money now and need not wait until the upcoming market day in order to enable the porter to save money.
Furthermore, we see from the first Gemoro that even if your victim brought the drink from his house and paid only fifty cents you still have to pay four dollars since the price your victim paid is irrelevant, just like the store owner is recompensed the amount he charges and not the amount he paid.
Another place where we see that the determinant is the location is the ruling of the SA (CM 101, 9) that if a debtor doesn’t have cash and therefore may pay back with goods, the value of the goods is the price they can be sold in the place they are given to the lender. Neither the fact that one can get a higher price in a nearby city nor if they will be worth more at a later date has any bearing on the valuation. The source is the Rosh (BK 1, 5) who writes, “We evaluate the goods at the price they can be sold at that moment at the home of the lender.”
However we take into account the price in a different location when one marries (kiddushin) a wife using an object that is worth a pruto only in a different place and not in the place where the kiddushin took place. Even though in order for the kiddushin to be valid the groom must give the woman the value of a pruto, and this object is not worth a pruto in the place where the kiddushin took place, nevertheless, the Gemoro (Kiddushin 11A) rules that we have to take into consideration the kiddushin and not dismiss it as being invalid because the object may be worth a pruto elsewhere.
This does not contradict what we proved since many Rishonim (Ramban, Rosh, Ritvo and others) explain that it is a special rabbinic stringency with respect to marriage, that due to extraneous considerations we don’t dismiss the kiddushin entirely. Even those (e. g. Ran) who maintain that this is a Torah law explain that it is because the woman can say that to her the object is worth more than a pruto, but not because its actual value is greater than a pruto.
An application of your question is a question about an event that happened in concentration camp. The story is written in the sefer, Bedidi Havei Uvdo. Rav Binyomin Rimmer, who later became a well-known Rosh Yeshiva, was very sick and weak and felt that if he doesn’t get something to eat immediately he will die. He went outside his barracks and noticed a fellow inmate running in the snow with four loaves of bread. In order to save his life, Rav Rimmer caused the man to fall, stole one loaf, ate it and survived.
Since the halocho (CM 359, 4) is that one is allowed to steal in order to save his life but he must compensate his victim for his theft at the price of the theft at the time of the theft, Rav Rimmer wondered how much he owed the victim since the value of a loaf of bread in concentration camp was far higher than in a store outside the camp.
According to the above, it is obvious that he owed the value in concentration camp. He asked Rav Chaim Kanievsky zatsal, who also ruled that way but brought a different proof. Rav Chaim cited as proof the Midrash (brought in Tosafos Shantz Sota 10B) that Avrohom Avinu would ask his guests to say a brocho after eating and if they refused he would demand exorbitant payment. When the guests objected to the exorbitant price, Avrohom Avinu replied that since they were in the desert the price was justified. Since Avrohom Avinu was honest, obviously this is correct.
Another interesting application is to the rules of overcharging, ono’o. In Otzar Hamishpot (volume 2, page 8) Rav Mendel Schaffrin was asked about the chassidishe minhag to buy a knife on Erev Rosh Hashana. Since many go to Uman for Rosh Hashana, several people bought knives in Israel for about five shekels and sold them in Uman for twenty shekels, which was also much more than the local Ukrainians charged. Rav Mendel ruled that this is not a violation of the prohibition to overcharge-ono’o since the Jews who come to Uman comprise an independent market and if that is what everyone in their group charged, that is the fair price.
In conclusion: You owe the value of a cup of the drink at the beach and must pay that amount right away and cannot delay payment until you return home if the damaged party wants another cup right away.