My AP1 mods aren't super extensive on the hardware side, and involve some hardware between the AP1 module and the car, as well as between the steering column module and the car, tied into a small computer (a raspberry pi initially, upgraded to a miniITX box eventually) to do some heavy lifting (map/GPS processing, path databasing, etc). The magic is in the software side, which keeps the car happy while augmenting everything as possible. Cost wise, it's not too terrible. Development time wise is a whole different story.
I tried to look into a way to do some commercialization of the AP1 mods I did so other's could benefit, but, the liability factor is just too high to be cost effective. Even releasing open source with massive disclaimers is just too much real world liability for me to take on to do so, unfortunately. I even floated the idea of releasing under an anonymous pseudonym, and was advised against doing so since "it would be too obvious."
I did find an insurance underwriter who would take on the risk, but the cost is just so astronomically high, and long term, that it just doesn't make sense to do so. As much as I wish I could release some of this stuff, even as-is work-in-progress form stuff, if helpful, I just can't take on that kind of risk at the moment.