Is there a price that is listed on the contract to sell the Chometz? If not, how can it be considered a valid sale? Who sells an item with a contract without a price?
You have a point. There are rabbonim that have a place on their forms to write down approximately how much the chametz is worth, for this very reason. This doesn’t mean that if it isn’t written that it isn’t valid, and many rabbonim don’t require it. In the contract that is signed with the gentile it is written that for now he is giving a down payment, and he is acquiring the chometz. After Pesach, he will have the choice to wither finish paying for it, and then the exact price will be assessed, or he can choose to sell it back. This way it is a valid sale even if they technically didn’t write a price.
Have a chag kasher v’sameach