Free to guests! I pay for the service of having it there, unlimited photos. I just started pricing them out but the cheapest I've found so far is $800. Includes fun props, hats, feather boas.. some include unlimited strips so if 5 people cram in, they all get a copy plus one for the book! Most include the guestbook for the bride and groom. It's all customizable with your names and date and whatever backgrounds you want.
I'm all into it but I have to realize its not about me, it's about what my guests would want to do. If people won't use it, it's not worth it. I probably won't even have time to use it myself!