If she is truly a close friend, I would be honest with her. I would make it a thoughtful gift--heavy in thought department but light on the wallet. Do they have children yet? You could always give babysitting coupons. If you have photos of the bride and groom you could always make the wrapping paper too just by photocopying the photographs. Do they like wine? There are some great ones that are in the under ten dollar range. If you are able add a silver ice bucket and/or silverplated corks to go along with it. Another great gift (I think) are the customized "branders." You can get them on Ebay even for a few dollars. If the couple likes to cookout, they can custom brand their steak and chicken. Just a few ideas.
Along the same line, you could get nice aprons from wherever (Williams-Sonoma, etc) and have them monogrammed with their initials. What about nice bath towels or heavy white terry robs monogrammed with a bottle of bubbly? The ideas are limitless.