Paint mega-disaster (long rant)

Damn that is EXPNSIVE! Personally I think you are SCREWED if you do not have some type of contract, estimate etc. Sounds like you just gave them a GREEN light to get the work done! If that is the case then you owe the bill whatever they say it is. I damn sure would sit down and see what could be done but based on what your side of the story is you gave the OK! Sit down with the Owner and discuss the cost!
keep a level head and be professional, no need in blowing up , cussing and getting all pissed off. That will just make things worse for you, because they still have your car. Personally I think your car should be finished for what you have already paid but that is only my opinion based on your description. You should "NEVER NEVER NEVER" have any shop work on your car without an estimate of some type,or some type of contract etc.
I would try to get the owner to settle somehow on a reasonable $$$ amount and then get your car the hell out of there FAST! Absorb your losses and go on. Take the shafting as a learning experience! If you are like me you would want to beat his *** real good and let him understand he just can't screw people over without concequences! But that is not going to do you anygood neither.Just settle for an amount!
Pay the bill and haul your car away from the crook. Then write your story and post it everywhere you can to make it seen and make his crooked *** be known and his business suffer as much as possible! Tell everyone you know and spread the word on how crooked his shop is and what they did to you.