I'd say, unfortunately, they're much like the AD&D hardcovers in value... only nowhere near as "common." So, probably around $5-10. To get the most up-to-date version of the game you'd want the 4th print PHB. It's very easy to tell print number -- look at the spine. Whatever number the product code ends with is the print, so TLGXXXXX1 is print 1; TLGXXXXX2 is 2nd print, and so on.

It might be a little more if it's a 1st print, but not really.