Sanguine I couldn't even tell you. I suppose it would depend on if ads will still be included in the domain part and if you do a tier structure. I would have to look at some other services and kind of compare and then make the options cheaper than the services that are already out there. Also, if you have a domain with another hosting company, would you be able to just punch in the dns numbers to have it forwarded to your host, or would the domains need to be hosted with you? A service like this seems like it would be very nice. Then you don't have to purchase a web hosting plan from those bigger hosting companies.
Sorry I don't have a better answer. There are so many options and a lot of things to consider when coming up with a price. Of course, once the research is done, maybe it won't be hard at all.