My take...I've been in the RMCA for over 15 years.
At first, pre-eBay, we just didn't know how common certain maps were, and there was a rule of thumb about how much we thought something was worth by decade. A lot of things got traded at Expos that we wish we hadn't, such as a non-graphic '30s map for a Shell license plate map. Then things came out of the attics, and prices tumbled. Now prices are starting to stabilize once again. Some of us who can't afford the big ticket oil co. maps any more are enjoying state official maps and AAA maps instead or looking for weird variants of common maps. The latter don't really lend themselves to display too well.
I can easily judge how much a map is worth (to me, at least), but photo postcards are another matter!
I want to mention that the RMCA Expo is in conjunction with the Indy Advertising show at the end of September this year, and it's a great place to learn about the maps, too, rather than just seeing them.