I cannot understand how GSM shield can cost 50, 60, 100$, although a full cellphone with keyboard and screen costs just 30$. I think the cheapest way to give Arduino some GSM capabilities is to directly connect it to a real, full cellphone. This could be done by implementing FBUS protocol libraries for nokia phones (does it already exist such a library?), but for not-Nokia phones? Could interfacing to the keyboard be a solution? As far as I know, a simple diode can be used as a switch, if you connect it in a proper way.
Could this work? Should I connect the "floating" pin just to GND?