I'm not going to try estimate price, but echoing what Recons said it could stand for 'Mixed Reality' which I'm sure you know yourself anyway. If you've owned it since 1988, i wouldn't even consider selling yet (I'm not sure if you are) until at least 2 years when Mixed Reality is in the public eye more and people are more aware of that term, what's 2 years when you've had it 28 years?
Back in 1988 can i ask why you opted for .net though? Looking at Whois MR.com got registered 1991, why didn't you go for that one?