Did his quote include anything else though? Not a new bezel maybe, but perhaps some other electronic equipment?
It depends on exactly what they wanted to do for the price. If they spend several hours on it and use no new parts, it's a total ripoff, and should cost about $100 - $150. However, if they do a lot of work, including new MDF panels, vinyl, fiberglass, etc. and take two day on it - it's a perfectly normal price. Anything in between should be proportional to the first two opinions.
I really have to believe that there are some aftermarket parts already available for you. It seems everybody has some big GMC thing with a modded stereo among straight pipes and a few other things. I think if you search you'd likely find something for $100 or so that you could work on yourself.
www.crutchfield.com is a good place to start. They don't have the best prices in the world, but thet are at least as good as any Best Buy or Circuit City. What this site does have is one of the most stellar customer service teams I've ever seen, and they include a ton of "extras" (instructions, adapter pieces, etc.) with most anything you buy.
If you can give a better idea of what you'd like in the end, I'm sure it can be figured out around here.