Seriously...who does this girl think she is? Does SHE routinely have $500 that she's not doing anything with, which she would be willing to put towards a party for someone else? If not, then WTH? That's a LOT of money, and awfully presumptuous to assume that her BMs all have $500 and want to spend it on her.
Now I would - heck, I HAVE for two weddings already this year - spend $500 or more on travel to the wedding, a BM dress, reasonable wedding and shower gifts, and other normal expenses that come with being in someone's WP. I'll even put up to $200 towards the shower and BP, if I happen to have extra money at the time. But even if I had $500 and no better way to spend it, I don't think I'd spend it on someone's BP. Heck, I wouldn't even spend that much on my OWN BP!
Oh, and every wedding-related expense should be discussed with the BMs BEFORE you commit to it! You don't say, 'You need to pay X' to your friends! You say, 'I'd like to do Y and it costs X amount. Can everybody do that?' (and if you're smart, you talk to them separetely, so there's no chance of one person seeing the others agreeing and feeling pressured to agree, even if she can't afford it).