I agree wth buttermilk.
If you cant afford a decent ($80 and up per person IMO) then buy a small gift and card
There have been wedding that we havent attended due to cost (bloody destination ones) but we have posted a small gift/card and then had the couple over for dinner at some stage.
I do think its rude not to come with a gift to any sort of function be it birthday wedding/christening/ housewarming etc. A card and a lotto ticket is not beyond the purse for most people.