When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I was going to tell you that a condensor is about $100 to $150, but when I checked my usual source - The Last Detail - the web address switches over to some web hosting site. Did a Google Search and it looks like the url is dead - hope they're not because they were a reasonable source for Vette stuff. Oh well - try the NAPA site www.napaautoparts.com or www.ackits.com or any of the Discounters for competitive pricing on parts.
To replace the condensor, everything has to be sucked out and it makes sense to put a new Accumulator in it too - about 85 Bucks. If the shop is any good, they will at least pull a vacuum for an hour (and make money on something else while the machine runs) before they dump the charge back in. Assuming you're staying with R12, it's running about $20/lb and it needs 2.25lbs. It will also need at least 3 or 4 ozs of mineral oil (assuming the Accumulator is swapped out too) and the last time I bought some, I think Autozone soaked me for 12 Bucks for a quart of it. Anyway, assuming you're getting a condensor, accumulator, gas and oil for $275, it's not bad, and an hour an half to do everything: install, vacuum, charge and leak check isn't out of the question. Further assuming an $85/hour shop rate, I wouldn't balk at $500 for the whole deal.
If they do all the things that SunCr stated, the labor doesn't sound bad. Shop around for parts prices though. I was in the AC section for my 85 on NAPA's website last month, I needed an accumulator, and it was $46. (BTW that was even cheaper than Advance or Autozone's website price)
Why not buy the parts you need, install them, and take the car to have a vacuum pulled and the system recharged? Probably save a few bucks, and you could parts shop even more competitively