Dollar per guess, a form of gambling?

Assalamualikum, My community holds monthly functions where all of us can get together. In these functions what happens sometimes is, either a cake (or similar) is taken around, members of the community have to guess the weight, or candies in a jar where we have to guess the amount, or a wrapped up object where we have to guess what it is. Each guess costs a dollar, which is for charity or the benefit of our community. At the end the correct guesser wins the object (cake/candies etc.), which costs more than a dollar. Sometimes the winner donates it to back to the organization (but not always). Is this permissible in Islam, or is it a form of gambling? Please explain answer if possible. Jazakumullah.