I don't give cash for weddings. Period. I understand that giving cash is the norm in the Northeast, but it's not the norm in the south, and doing so makes me uncomfortable.
I give exactly the same as I would give if I didn't know the bride and groom wanted cash -- a present in a price range I feel comfortable with.
This is harder if they didn't do a registry. But maybe there is a registry for shower gifts, and you can get something off of it? If not, I'd give a generic wedding gift -- crystal, probably. I got some beautiful crystal from Tiffanys for my wedding (that I hadn't registered for) -- a pitcher, a vase, etc. Something like that would probably be in the right price range, and if they really hated it, they could return it (that blue box makes it pretty obvious where you bought it!).