When were X-based eames chairs produced?  

Lots of info available
There are tons of postings available about the dating of the Eames shells on this forum. Searching the archives may give you a much more accurate & throrough picture.
Larger shockmounts and x-bases are usually indicative of earlier chairs, altho I cannot say specifically when without checking a reference book at home. As to value that is a rather subjective subject. Lots of factors come into play: condition of the shells, color, age, markings, coverings, originality of the bases, location, demand in the marketplace. Eames are desireable in the NE area, but they are also fairly commonly had. I know dealers in Cambridge and Boston who don't really bother making a big deal out of the shells because they're so easily found on Craigslist.