What does questioning his integrity have anything to do with the fact that he is not an outgoing person and is perhaps, financially troubled?
200 bucks or so is not simply a 'few bucks.' You sir have obviously no experience with earning money, and also hold no regard to the fact that not everyone has a million dollar sugar-daddy they could live off from. Your comments really did do your image as a shallow and ignorant guy. It is your integrity that ought to be questioned, not his.
Also, what does being an extrovert has to do with your virility? There's absolutely no logical correlation within that.
No school should force their students into events, unless it is to do with learning and thus no excuses are needed. I don't actually think that you need to physically do anything, but if you have to, just tell them the truth- the formal does not interest you and that you are also not financially at the best position to attend. Tell them the truth and get over it.
Personally, I didn't attend the formal. I believe that its a waste of money and unnecessary, going as far as to Carlton at nigh, hire limos, suits and the event money by itself just cost a lot. My parents actually wanted me to go, and they are also willing to pay for everything but I said no because I can see better use with that money and time. That's just me. And for the friends bit, a couple of casual barbecues, night outs or lanning parties are good enough for me. You can have a great party in casual, simple wear, and the tux just doesn't cut it for me. But that's only my opinion anyways. Obviously, I am sure that people enjoy dressing up in a suit at times and find the whole experience of it rewarding. And I have no problem with that as well.
Just don't make your belief/interest a need for others to abide by. After all, we are all very different people.