Research craigslist.com and www.motors.ebay.com. Those two sites should give you a good range of prices. Since the anniversary edition is a bit more rare than the production models (K and F, in this case) expect a bit of a premium, but not like you'd think. I've been seeing a lot of the Limited models lately, but I've been looking for them.
I have seen anniversary models with 4 into 4 and 4 into 2 exhaust, but not a 4 into 1. If it's a name brand exhaust, that probably won't detract from the value. Some of these came with fairings, which most people removed. Not sure why: I would have liked a fairing on mine.
I hope this helps.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.