I am working on a similar service for my own needs (and some other friends). But, my current plan is to hit $100/yr or so but the hardware is included in that cost. I am assuming $2/mo for costs of the number and minutes and retail costs for VOIP hotel phones is about $50. Hopefully the hardware costs amortize out and then I can offer a discount to users on subsequent years.

(If you want to go DIY you can save quite a bit of money too- I provided some breadcrumbs for doing that in my comment on the parent)