My 16 year old son lost the money for his swim banquet?

I gave him $70.00 cash (that's what they required) He took it to school and put it in the captains locker. He put it through the air slots. Well guess what, its not there. I don't have a clue if what happened to it. I can't imagine either my son or the other boy took it. Anyways I told him I wasn't paying 70.00 again. I think he should offer to pay. I told him if it isn't that important to him that we are there (his family) then it shouldn't be so important to me. Am I wrong? I really won't to be there but I also want to teach him a lesson about responsibility. I can afford it, so money isn't really the issue. Help what do I do? He just spent the only $100.00 he had last night. He took a girl to dinner and the movies.