Well, it depends
I just mounted a 50-inch plasma, using my friend and neighbor, who is quite handy. We did it...but it took about seven hours together (14 man-hours total) to plan it out, find the studs, drill the holes, mount the bracket, run the wiring through the wall, run the wiring under the house to the amp, then mount the TV. It looks fantastic...but what do you value your personal time at? For some folks, it is far cheaper to pay a pro $400-500 to mount and wire the thing. For others, it is a matter of pride and economics. I'm not sure I'd do it again! (While my friends were out on a Saturday night getting drunk and partying, I was under my 80-year old house, straddling a sewer pipe, drill in hand, boring a hole in the floor joists, praying, "please god let this be the right place!")