I think it all depends on what you plan to do with it. If you plan on riding it, just the way it is, and want it clean and running good, then 1500 is a fair price for a nice running bike. If you plan on going through it anyway, and or converting to a cafe, then you should look for something for less and maybe not worthy of a Restore to original. In my case, I started looking for a nice looking running bike to covert to a Cafe, and the more I thought about it, I am getting rid of the seat, the forks the tank and cutting all tabs off cutting the frame and altering it, changing the wheels out, etc, etc. So what does that leave me with? A frame and motor, LOL.... SO I started looking for something that I would just run, maybe not running at the time of purchase, but I know I can get running. I ended up buying 2 CB550's for $300 for both. Both I know would run, one had everything and the other missing a lot, but had everything that I wanted. I one running a couple days later, then tore it down to the frame...
So, like I said, it is all what you intend for it! If you want an nice looking CB550 that runs, and that is all you want, but maybe make it better, 1500 is a great price.
As for reliability, I have heard it over an over about the 550 and 750, that they are bullet proof. The one that I got running in a day, hasn't been started since 1989. I haven't tried the other yet, but I know I will get it running, and that thing hasn't been ran for over 25 years. One thing that should be noted, there isn't much to these motors, so, if something does go wrong, it is fairly easy to figure out what it could be..... I would suggest upgrading to a electronic ignition and get rid of the points for added reliability....