I am not sure, if this can help you. But I found some project using Nokia's 3310 model mobile as GSM Modem. And from Elektor Electronics magazine I found some project using readily available GSM module, for under some USD50, as much I remember (but the supplier was UK based, so a lot of VAT & such things are added up. If you can find the OEM, you might be able to bring it enough down). May this show a lead to you or this info might not be useful at all. In later case, sorry for wasting your time.