You can do it depending on the host.If you use an overloaded VPS on some of the cheaper VPS providers out there you might be only able to host 1 or possibly 2. Sometimes you cant even host 1 on the lower end VPS after so many people join. I Had 1 VPS that I could fit 5 people on but after that it became unstable and the Load would be very high and crash the server. and this was a 15$ a month VPS Server. Honestly It would be hard to make money on this project unless you found a very good non overloaded VPS that was cost effective for your budget. You can easily host a 50 man server on 1 VPS. However most people do not need 50 man Servers they need 10-20 and for each instance of mumble the resources will become greater than just more users connecting. This is why most people get a Full Dedicated Server and see how many instances they can run on 1 Server without the Load going high and then see if it is cost effective in competing with other VOIP services out there.
The overheads on mumble are incredibly low, if you are crashing a server with 5 people then something is very wrong, but not with mummble.
As GameFrame says, the issue is the setup, not that it's hard, just a pain to do it manually every time. I have automated it for our use however I am really not sure if there is a commercial type installer out there you could use, perhaps someone else will know.