Just go to
http://www.gemm.com and look at the prices being charged by reputable sellers (ones with 4 or 5 stars); that is a great indicator of market value. They have search filters, so you can just look for vinyl. If you select "use classic result format," it's a LOT easier to read.
http://www.musicstack.com is very good too, and easier to use than gemm.
http://www.popsike.com is a good site which lists completed auctions, so you can see how much certain items have fetched on ebay or other auction sites, but gemm and musicstack tend to be better indicators of real market value in my experience.