Anything where you want a phone number and to communicate with the rest of the phone network, I'd expect to pay money for it. Google Voice is the (only?) oddball here.
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)
Makes sense to me. Ideally, people outside of my friends and family would not be able to call me on my regular phone either.