Appropriate Monetary Wedding Gift??

I'm attending a wedding tomorrow night with my family (there will be four of us). The bride is my best friend's sister and the groom I hardly know. The couple lived in Seattle and now Kentucky, but the wedding is being held in Michigan where the bride grew up. What is an appropriate wedding gift to give if we hardly (if ever) see the couple?? It's my best friend's sister; however, even when she lived in Michigan, we rarely saw her. The wedding is being held outside in the garden of a banquet center and the reception is in the banquet center. Food is buffet style and I know they didn't spend a ton of money on cake/favors. Any suggestions would be great! (But please no whatever you can afford!)