I am currently working on a ground station app for iOS. The problem I have is that I do not have an APM yet and with my college student budget, I can't afford to buy one new. So I am looking to buy a used one so that I can better understand the MAVLink protocol and test out my current code.
I can afford to spend $100 at the moment and I can't really go above that.
You could replay TLOG files - they will help you some what until you get an APM.
You could probably get an old APM with a 1280 too - that would be enough just for messing with the protocol