Ask HN: Charge money on top of SMS?

I have what I think is a VERY promising idea for a (geo-spatial) SMS-service.

Or at least it is promising in the sense that people would want it, however making money is always the problem :)

But if customers send an SMS to my site and I send one back then I can charge money for this service right?

Would it be reasonable to charge 10cents for an SMS that costs 70cents to send or what kind of profits are good to start with?

What services/companies do you recommend for this?

Why isnt twitter doing this?