No, it does not seem as though your plumber is ripping you off. An expansion tank is used to provide protection for domestic hot water systems and water heater systems. Basically, it protects the system from developing too much pressure. The tank contains air that absorbs excess water. 
Basically, all water heaters should have one in order to function properly. However, if you feel as though your plumber is trying to rip you off, it could not hurt to seek a second opinion. Chances are, another plumber will tell you the same thing.