Depends on the length of your relationship and kind of friend. If this is a really close friend and you know exactly what they like, might buy them that instead of cash. Again if they are your close buddy, they wont care for money, might make a collage of old pictures of you with them. If this is an acquaintance I'd say $50-75.
I agree, it depends on your financial situation, but if you are known to be in a good situation, you are probably required to pay way more then someone who is not. I would say about 200 - 400 dollars would be a good starting point.
A good alternative to money for an acquaintance is to simply just search the bride and groom's name on google followed by the words 'wedding registry'. This should heed some results on sites like Bed, Bath, and Beyond, Target, Amazon, or Crate & Barrel. In America it's commonplace for to-be-wed couples to register on at least one registry. Registries are a list of items the couple would like as gifts. Then you can simply purchase something they requested- which eliminates the need for money or the 'What should I buy?' guessing game.