These sort of games can and do make serious money.

One thing to keep an eye on though is that some payment gateways are moronic when it comes to taking payment via premium short code sms.

I looked into it for a role-playing game some years ago and several had serious holes in their api.

For example one of them had a system where you submit a form that then sends an sms which subtracts a set value from the receiver once they reply.

The problem was it would have the sms value in dollars in the form with no checks or validation - simply using "inspect element" allowed adjustment of the fields and no hammering the point home to their tech guys would make it clear it was a security hole.

Make sure you choose a reputable, reliable and honest payment provider!