You're not being ripped off unless you pay it and even then, if you agree to it, is it a "rip-off"? With that said, $500 is too much. I would expect $100-250 max.
In addition to xlnt advice already given, I would suggest the following:
1) Posting for help on a local hummer club forum if one exists. I belong to a local on-line club that specializes in ford diesels and have used them for much help - both on and offline. Good to start an association with local hummer owners anyway if you are wanting to do mods/trips/etc. Be willing to pay something, buy beer/dinner, etc.
2) Advertise for help on CL.......but with the following caveat. Ensure the person can show some knowledge/experience in what you are needing done. Anyone experienced can bring the few tools needed to do this, do this in your garage/driveway and will not have the ovhd of a "business". ANd with today's economy, there are qualified people out there who could use a few extra $. I have done this recently when I had problems (and needed a specialty tool) on putting a motor back together and found a certified ASE/Ford mechanic who lived close, could do the work in my garage and needed some easy cash. Win-Win. Know what you are willing to spend (I advertised and paid $50 for what would have been min. $250 elsewhere plus I had to get the vehicle there). But do a little research/verification on the person before you let them come over or touch your vehicle. I got this individual's name, a reference and was able to on-line verify he was indeed a mechanic.......and I asked for ID when he arrived and quizzed him about how he was going to do the work (I already knew what needed to be done, just needed verification, help and the tools). Likewise, I was present the entire time he worked on the truck. Since then, he's become a good friend and resource.
For me, part of the fun of vehicles is working on them yourself. Very few of us can do everything (I can't) but the more you work on it and learn, the less you will be at the mercy of others. Whoever does it, at least watch and learn.
BTW, here is a thread showing an install:
http://www.pirate4x4.com/forum/showthread.php?t=756631