I agree with your sentiment about your friends being horrified if you spend too much money. They are probably just happy to be able to share something good with you. If they said they would like a bottle of wine, then a nice one ($30 and up generally gets you something good) or if you want to spend a little more a nice bottle of aged port or liquer instead. Or just get a bottle, and perhaps some flowers or chocolates or something girly for the hostess too.