There are a few factors affecting the price of maps I would imagine. From the type of copyright to the obvious amount of hours put into create the work. I would expect good, deep, maps probably rival or surpass centerfold paintings in terms of work hours. Simpler unpainted maps probably a lot less. If you are commissioning a piece I would imagine it really depends on what you are asking for and what the artist estimates it will cost in terms of hours and of course adjusted according to the copyright.

Perhaps if artist were to post a map and post the price they would have charged for that specific map it would be better than some sort of general list. Really it's the only way I see of doing it. I can't imagine a high quality map costing as little as 50-60 dollars. That really seems like a bargain bin price for even relatively plain black and white map.