If you could only afford $10 then that would be a suitable amount. $50 is more than enough if that is what you can manage. I would never judge a friend by the amount of money they gave me! i am sure your friends wouldn't either.

As other people have said, you don't HAVE to give money. If i go to a wedding and they ask for money but i have seen something i would prefer to buy them, then i buy them a gift.

Again as other people have said, presence not presents is important. And if presents are more important to the couple than presence i would seriously be thinking about my friendship!