Hi Hans,
Yes, 50 KB that's around the price... Depends on condition, service and... the seller. Odometers, well, don't trust it so much...
Just listen to the engine (when hot, no kling-klong!) and check the smoke (no smoke of any color when hot engine!) and the leaks around. If you can, check the clutch (no slip) and the brakes: riding the bike gently (40/60 km/h) be light on the handle-bar and brake fermly front and rear brake. The bike must brake quick and straight without trembling (you feel this in your hands and "butt")... Then check chain and sprockets, and all the electic devices: must work!
True advice KZ, but I should say go up to 200/250 KB to be "quiet" with a "modern big bike"...
The CB1 production stopped in the year 1993, so these bikes are older than 15 years now. You can't find a CB1 quoting 100 KB or more... It would be a scam. The touch is betwenn, say, 40 KB and 80 KB for a "top-very-mint-amazing" one, not more.
The CB 400 came later. Good bike too, but the technology is not the same. Not same frame, not same engine and so on. Prices are usually up, depends of the year...
Here (spanish web site):
www.cb1.cun.net
you can download for free the owner manual and the service manual.
As says Silver Hawk:
www.hondacb1.org is a friendly site.
Hope that helps,
Cheers,
Gobs