I've been saving maps for a long time and have accumulated thousands through many outlets- flea markets, tag and estate sales, swap meets, gas shows, auctions, friends, etc etc. They are a relatively lower cost (with exceptions) gas collectible. You can obtain examples from obscure companies that have higher priced items (cans, globes, signs) that may be out of reach price wise. Maps are neat little windows in time that can show what equipment, signage, products were in use at those stations.
Maps can be tough to display. Many map collectors preserve their maps in archival grade plastic bags or sleeves, and can frame or store in 3 ring binders. Others prefer to display in road map racks that were used in the service station. Most collectors are fussy about condition and will pass on damaged maps.
Maps can be had cheaply in bulk lots, averaging under a dollar each. A majority of maps are valued between a dollar and 30.00 each, depending on brand, state, year, collector preference, etc. There are also maps worth more, some reaching into the 100's of dollars each.
I recommend anyone interested in maps join the Road Map Collectors Club. They put out a nice newsletter and have a national convention. Contact them at www.roadmaps.org