I wouldn't pay stealer's rates for this. Plenty of competent a/c places out there.
However, the selling dealer is liable for defects for the first six months after purchase. The law states that such defects were likely to have existed at the time of sale & therefore liability is on them. I'd also point out that (if you got it from a VW dealer) their "120 point check" or somesuch cr*p that they always claim used cars get, wasn't thorough enough. I really think that a case of polite but firm could get them to gas up your a/c without charging you.
Man, if I'd paid for a car from a dealer that had crap a/c, it would go straight back.