I personally voted $10k. I have ~10k in referral credits and would give em all to get autopilot upgrade. I'd probably pay same out of pocket, but would need to wait to collect the cash. Personally, even though trading in and purchasing another would get me a new car, the hit I'd take on Mileage (72,000 Miles) basically kills my trade in (tesla already wouldn't give much) and private party is beating a dead horse. so for the cost of a brand new sub-compact, it would bring my car up to current.
The problem with this poll is that one can be completely unrealistic. A retrofit is likely in the $15-20k range, if not more. New front and rear bumpers, new windscreen with new camera in the mirror assembly, new radar, new ultrasonic sensors, new wiring harness, and so forth. The labor alone would be in the many of thousands.
Wouldn't upgrade to just get the limited capability of lane keeping / changing autopilot and TACC. There's been so many updates, I'd rather just trade the classic S in and upgrade. Everything from folding mirrors, parking sensors, rear heated seats, heated steering wheel, autopilot, "D", next gen seats, LTE upgrade, LCD upgrade and more were introduced since 2012/2013.
You obviously have it because all Tesla cars after Oct 2014 have the hardware. In case others are not sure, the rectangle thing in this image is the front radar. Any MS that has this, has all the autopilot sensors.
The AP stuff, with maybe the exception of the emergency braking, does not interest me, so I have no interest in upgrading. I think the other thing to bear in mind is all the other incremental changes made to the car over since pre-AP days--at some point it seems to make more sense to just trade-in and get a new car.