I agree with Runsongas. The MK10/G250 is a great combination and you will be hard pressed to find an "upgrade" that is on par with what you already have. As far as "what is it worth?" It's worth a lot more to use it and avoid purchasing a new regulator. To get an idea of the amount you might expect to receive should you sell it, go to eBay and modify your search to include only like items and click on "sold listings" under the "show only" section.
Re the age of your regulator; I used to work in a SP store. If my recollection is correct, the MK10 did come out around '84, but the G250 did not appear until '86 and according to
http://www.scubapro.com/en-US/USA/regulators-timeline-eng.aspx the Graphite around '87.
Keep what you have and go diving with the money you save.
"That's just my opinion. I could be wrong." -Dennis Miller.