Well, right, but we're chasing the constantly-morphing set of requirements here. Are talking about friends coming to visit your house, customers visiting your business, or what? I'm trying to imagine what business has a business requirement for an insecure network to keep their customers happy that wouldn't be served by setting up an open AP with ONLY internet access.
In any case, I'd inform your customers that they have access to a WPA-secured AP if they want it, and that it only connects to the internet (they can't browse your business LAN or share your printers), and if they can't afford the $50 upgrade they probably aren't going to be worthwhile customers. 8*?