If you get the AC done at a shop then your looking at $1000 out the door. They are going to properly evacuate the system and replace the compressor and dryer. Then refil the system, check for leaks and such.
If you just find a used compressor and do yourself then I would say under $300.
I just got the car yesterday. And today, the check engine light came on while I was driving. The car still runs normal (no oil leaking, no noises). I'm going to get a code check ASAP.
I have to note though. Before I bought the car, the AC Compressor wasn't working. It started to make the belt burn and smell. The owner put a smaller belt (leaving out the compressor), but the smell is still there. I'm not sure if the process of changing belts messed up something.
Previous owner would only bypass AC for two reasons I would think. Siezed compressor or one with bad clutch/noisy bearings. I would not put a belt aroud the compressor if the previous owner bypassed it. Wait until you snag an AC compressor and then try it.
Give a good look to the belts that are on now. Check for fraying or melted spots. See if you can see exactly where it's rubbing. Maybe take the belts off for the inspection and rotate all the pullies by hand to make sure they are spinning. Lastly look closely at your harmonic balancer(crank pulley). Make sure the outter ring is still fully secured to the inner. Seperator of the crank pulley is very common on nissans.
I think if it comes back then the solenoid is bad. I don't think cleaning that will help. Reason being mine isn't even hooked up and I don't have a code. But my solenoid valve is still there and plugged in. This leads me to believe that the early p10's don't look for EGR flow or temp. They are just looking for an electrical connection on the solenoid.