I would put aside the various emotional issues at play here as a starting place. It's certainly not that I think they're unimportant, but I think they may be obscuring the more important issue: How much can you legitimately and reasonably afford to give her to help with the wedding? I think that's the best number to go with. If that's $500, then give $500.
But here's my feeling about the emotional stuff: I try to only give gifts if I can give them freely--without reference to any emotional baggage, guilt, hard feelings, etc. If I can't give them freely, I don't give them at all. If you really are at a place where you can't give your SD a gift, then don't give her anything. I think that's a legitimate and fair approach if that's where you are emotionally. But be sure you're really there before making that call.
Good luck.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher