How many miles on it? I would love to have a 79 650 but am facing a move and hip surgery which will be requiring some time off the bike and rehab and the surgery itself won't be cheap.So, my pennies are tied up. Fresh tires aren't cheap, new rear tire for my VStrom is exceeding $200 now. So, that increase the value, not kidding. A bike that needs little to nothing and a vintage bike at that is valuable in my book.
With inflation I cannot venture a guess and even though the 650 is a redheaded step child of the CB fours I think they should go for good money. I'm seeing auctions of CB550Ks in stock trim that are going for $4000-4500 for very nice examples and $3500 or little higher in good examples... But, exceptional bikes can push $6000-7500 for people in a bidding war. Not having a stock paint job is a strike for many buyers though. The fact it is stock other than paint and a nice running example everything else is gravy to me... The 650 can spank stock cb750s if breathed on a very little amount as Honda got so much right on the motor. If you are adventurous you can put a Dyna Electronic Ignition on one as we have a couple members who had done this.
I thought I bought a CB650 motor but it got resold after I paid for it but had not went to pick it up. At least I didn't buy more for it in anticipation than a few things, a Honda OEM head gasket being extremely hard to find and it is an expensive item when you do find it as they aren't available any longer and aftermarket support is even trailing off. That will be the challenge for owning a cb650, is talking Vesrah or Honda into making parts for them. They are getting parted out way too often, Mr Monkey claw seems to have lots of 650 motors from various years. I would hope they aren't parting viable bikes but I can bet they are as they don't try to sell too many whole bikes...when they do, they usually are getting way more than they would from private sales.
I would say $3800-4500 would be my expectations on the low end. You gotta remember the low numbers and one year production in standard frame is going to make them worth a lot more to collectors not too far down the road. Inflation is killing everyone's pocketbooks but collectors have the money when these kinds of deals come along. Soon people will be competing for a bike you want as a rider for a collection.
Haggerty will insure them for what you value them at if it is within the realm of realistic.