Not sure how different it really is.

I have to admit, as someone who is into quads (and embedded software) my first thought was that it would be cheaper to buy a $35-60 CC3D or knock-off dedicated APM module and then just add a Raspberry Pi to the quad as a separate unit. Having the two communicate is a bit more complicated but totally doable via serial if you're willing to hack on APM or OpenPilot code and a lot of the use-cases people are bandying about don't actually need the two units to communicate anyway.