I have an application which takes a basic input and provides an answer
to a user through a web page.  I would like to know what reliable
providers i can approach to setup an SMS number that a user can send a
question/code to, have that input directed to my application, and then
have the result text sent back to the user immediately.

I would also like to know what operating costs i can expect so that
the price for these "premium" messages can be structured to cover
this.

An analagous system to give you an idea would be a user wanting to
know the current temperature, sending an SMS to a weather bureau
number containing their postcode, the bureau processes the data and
provides a result, which is sent back via SMS.

Gramercy,
ash