My RSX was around $550. However, I added 4 interior sirens (about $20 each) and a backup battery (don't remember how much that was). Also, my friend worked at Ultimate Electronics and he gave me a little discount. Nothing major but something. I was figuring around $850 for the AG5 but decided to save the $300 instead.
In all honesty, go talk to a high end installer. If it's somebody that NEEDS the business they will tell you anything, if they could do without it you will most likely get more accurate information. I ended up not even going with the guy that sold me on the RSX because I could get it better with my friend, but the guy was giving me a no BS answer to my problem. He took into concideration where I lived, where I worked, what I drove, my wheels, car, engine, add-ons, everything, then made a decision on what he would reccomend FOR ME.