Not going to speculate on the knife's value based on Forum rules but am curious about its date of mfg of your knife.
I assume you know how long you've owned the knife and am not going to bother trying to dispute that but your statement does NOT square w/the available info.
While there apparently is no chart to determine the date of a Mark I from the serial # on it, there is a very detailed date/serial # chart for the Mark II's.
See:
http://militarycarryknives.com/MKDATETABLE/MkDateTable.htm
You'll note from the chart that Gerber did NOT start using a "letter" in Mark II serial numbers until 1982 and based on the chart, your Mark II would have been made sometime in 1991.
The Mark IIs that are worth the most were made during America's involvement during the Vietname War between 1964-1973 but pre-letter versions of the knife are also worth a lot. Lettered versions not so much.
Your best source for current price info for the Mark II would be on EB, ACK or other sites that provide the current results of sale of all of the knives sold there.
Unfortunately, many sellers on BF have the "habit" of deleting their asking price (which is not necessarily the selling) price from their sales listings after the knife is sold. So, you may or may not be able to see what that knife has sold for by trolling old sales ads here.
BTW, I have a Mark II w/a SN w/only numbers that was made in 1980 which I bought off of EB a couple of years ago.
