I am going to have to buy a second hand car… in Hawaii, where AC is kind of cool to have (pun intended). I am looking at Craigslist ads and virtually all of them will have a line which describes the car’s AC, as in “great AC” or “Cold AC”. Occasionally there will be the ad that says, “AC needs fixing”. What kind of gamut of prices could a repair like this run to? From x$ --> x$… Ok, kind of a vague question, but what’s worst case scenario? I am looking at Hondas at the moment. Comments and admonitions welcome.
Couple hundred to a couple thousand, depending on why it’s not working. Plus it’s usually not an immediate diagnosis. Unless there’s something blindingly obvious, what usually happens is they’ll recharge it and add a tracer dye. Then when the A/C stops working well, you come back and they inspect everything with a black light. If you’re lucky it’s just a gasket/o-ring or Schrader valve. If you’re unlucky it might be a crack in the condenser or evaporator, or a failed compressor.
And don’t ever fall for the "just needs a recharge " line. Refrigerant is supposed to stay put, if it’s gone it’ll just leak out again.
Faulty AC can be caused by a leak in the freon plumbing, a bad compressor or a fault somewhere in the control wiring and switches.
A leak can be trivial to fix if it’s in an accessible or inexpensive part. It can be “unfixable” if it’s in an expensive component or one that’s difficult to replace, as the inside evaporator often is - not worth fixing because the cost may exceed the value returned for a used car. But a good shop should be able to do a pressure test and determine the leak location - often a cracked hard pipe under the hood, or a bad seal on a connection, or even just something loose - for a reasonable fee.
A bad compressor can be expensive to replace - $200 to $500.
Wiring and control is usually inexpensive to fix and a shop can trace the problem quickly. It can be tricky, though - I spent two hours carefully diagnosing my Odyssey’s sudden “blow hot” problem and finally gave up. The shop found the bad relay, right there in the fuse box, in ten minutes. I’d tested it, but not the right way. $50 lesson in thoroughness…
You should always have a used car checked out by a shop before committing to the purchase; $50 or so for a diagnostic is the cheapest insurance you can buy in that case. If the AC is bad, ask them to specifically check it out and give at least a good guess on cause and cost to repair, maybe even that preliminary pressure test.
… or it could need the belt tighten a smidge …
Chances are, the AC problem in a craigslist car will be costly to fix. If it was cheap or easy, the owner would have done it. No one wants to drive a car without AC. If it’s being sold with a broken AC, it’s because they had it checked out and it cost a lot to fix. You should budget $750-1500 for the repair and the purchase price should reflect that discount.
depending on the age of the car, most accessory drive belts are automatically tensioned.
Best case is maybe a hundred or less, to recharge a slow leak or replace a fuse. But in such cases, the seller almost always takes care of it because the car will sell for more with the A/C working.
Worst case could be a few thousand, due to multiple problems. On some cars a compressor replacement can run close to a thousand (compressor, drier, flush, recharge). On some an evaporator replacement can run around 800. An automatic climate control head can be a few hundred. Et cetera – there are a number of different components in the A/C system. Double everything if it’s a European car.
Even with working A/C there could be a problem, like an expensive-to-fix leak that empties the system in a couple weeks – long enough to blow cold until the car is sold. If the A/C is not working the odds are pretty slim – closer to none – that the fix is anywhere close to cheap.
There is a device that keeps the belt tension correct … which may break or be packed full of mud … or the belt could just be wore out …
However … I think filmore’s point is spot on correct … the sellers would have fixed such a small repair before the put it on Craig’s List …
Many times it is not a big problem…
But mechanics will charge a small fortune for even the smallest problem. Expect to pay at least $600.
I fix these things myself and the cost is not more than $30/$40 many times. However I’ve spent probably $400 on the tools to do this work! (A/C only tools.)
If you can’t fix cars yourself, best to buy one with a warranty.
A trope but not always true. My shop “gouged” me for $70 for a A/C diagnostic and basic service, and did not charge additionally for the $10 relay that was bad. I don’t have any exceptional relationship with them and they are usually a bit expensive… so.
The ads that say the AC has a problem are at least being honest. If you look at a car that the owner says works fine, make sure you take a longish test drive and run the AC at max the entire time. I looked at a used car many years ago that the owner said “runs perfectly”. First thing I did was turn on the AC and got nothing but hot air. Owner looks sheepish and says, “Oh yeah, that doesn’t really work very well.” Translated: “I’m a liar and you’re not a fool.”
Here’s a somewhat relevant thread talking about the economics of used cars with known but undiagnosed problems. Might have some useful tidbits for the OP’s decision on bargaining or passing on such a car.
And what field are you in?
Yeah, really. Got a problem with plumbers, electricians and other tradesmen who charge an hourly rate? It’s called a living wage plus overhead.
Ah yes, the widely unknown and under-utilized Craigs List Warranty.
I don’t see what that has anything to do with?*
Might try asking what my knowledge/experience is!
It has everything to do with it as you seem to have this weird idea that no one should be paid for their labor/time/knowledge/experience/materials. So he wants to know if the same should be said of you.
It also has to do with it as GaryT has been an auto mechanic for quite some time (sorry Gary but I forget how long 
).
As I recently said in another thread on another topic …
Cynicism is false. It provides an excuse to expect nothing but the worst from others while simultaneously excusing all the worst in yourself. It’s lazy, selfish, and ignorant all tightly wrapped into a toxic ball of nasty.
Forty-three years and still doing it professionally. Can’t imagine why you didn’t have that memorized. 
When you are buying a car and don’t have a firm diagnosis I think it best to assume the worst, if you are a gambler maybe assume 50% of the worst case scenario. Something I do is look up the vehicle in question and see if I can find some common problem with that vehicle. There is a slight chance you may be able to check that while inspecting the car. Wet spots on the lines or at the connections might tip you off to a freon leak as the freon is mixed with oil.