I'm always amazed that every time someone asks this question, the answer is different. I asked this not too long ago and a lot of people were telling me to give around $200. (Then again, I also asked on LIW, so I'm sure I got different answers over there...)

Since I am in the bridal party (and it was a good friend with not a lot of money), I gave $150. I'm NOT a big fan of e-parties at all and normally would not give cash as a gift. I called this e-party the fundraiser b/c they seriously just wanted to get money to put toward the wedding -- or at least I thought it was going to go toward the wedding. They went on a couple of shopping sprees after the party with all the money they got. Granted, their money to do what they want with it, but... had I known it wasn't going toward the wedding, I'd have given less.

At any rate, give what you're comfortable giving. I'd never give more than $100 personally - and I'd probably give a gift instead of cash.