I'd like to build a prototype of simplest possible GSM device - a simple modem with SIM holder, capable of logging on to the SIM's provider network and nothing else. It's meant to be used as a GSM localizing beacon in larger application. It's meant to be as small, as simple and as cheap as possible. Being crude or rough doesn't matter. I'm aware of >50$ GSM modules/modems, but they are too big and too pricey, and I'm looking for a better solution. I'm also wondering if the cheapest way is actually through the disassembly of an old mobile phone? I can get one for about $10 (or even for free from some junkyard) - the only problem with them is that they have a lot of unnecessary components and board space that's difficult to remove without damaging the GSM module.
Also, is it possible to craft such a device with some simple parts/engineering samples?