here's a site that is subscription:
http://www.artprice.com/There is some very limited information you can get without a subscription, and I have been very pleased with the quality, but I would recommend buying at least a limited subscription if you have many paintings.
Before you do that, you might want to "google" the artists - some very lovely paintings are by relatively unknown artists. The site above deals with those artists who've had pics auctioned, exhibited, etc. KB
