C.,
It doesn't matter if she made it clear she wants money. The bride, for all of the other entitlements generally granted her during this special occasion, does not get to demand money from her guests.
Separate your feelings about their lifestyle, to the extent you can, and then ask yourself whether or not giving a gift is something you *really* want to do.
If the answer is no or if you truly cannot afford it, then let it go, and don't continue to second guess yourself.
If you decide you do want to give something, how about a gift card to Lowe's or Home Depot? Easy and inexpensive to mail. If they own a home (or even if they rent), there's always some home repair or project, so it would be a thoughful and useful gift.
If you go this route, decide what you can afford and don't let anyone try to guilt you into thinking it isn't enough. The mere act of taking time from your day to buy and send them something is more than enough. If they are so ill-mannered as to dismiss your thoughtfulness, you have to ask yourself how much energy you want to continue to put into any sort of relationship with them.
Hope this helps.
J. F.