How much should it cost for a plumber to replace a garbage disposal?

I had an old 1/2hp garbage disposal that died, so I went out and bought another one for $199. It's a 3/4hp one. The plumber is installing it now for $250.00. He says that this one is bigger and so some of the plubming needs to be adjusted. Still seems to high. It is what it is, but I'm wondering if I being ripped off :) Thanks.