I have a Mesa Nomad 100 head and I know Mesa/Boogie produced the Nomad series for only a few years.

Anyone know if an amp like this will gain value years down the road if I stays in good condition?
Probably not. The Nomad wasn't top of the line, and it wasn't received particularly well. Maybe in 30 years it will be worth a little more just for being old and a recognizable brand, but you're not going to turn any significant profit on it.