G'Day Geety, well I paid Andy Morris in Long Island $3000.00 for the January 1970 build K0 in my avatar, the engine is strong with only 25,000 miles on the odometer, and I really love riding it with the (un-muffled) HM300 pipes, it is great fun, and when I ride it, I can imagine what it would have been like "back in the day". Essentially, it's a time machine.
Like most K0's around nowadays though, it's a mixed bag of original K0 with some K1 and maybe K2 thrown in for good measure. It came with later carbs (common upgrade back in the day) and airbox, K1 seat, K1 forks and front fender, K1 fuel tank, K1 shocks and some chrome after market chain guard, so all I really got was a rolling chassis with the correct engine and frame numbers, with the hard to find gauges and sidecovers (cracked but fixable) basically.
But over here in Oz, K0's are not as common as they are in the US, in fact, there's bugger-all around. I was lucky that I had previously bought a set of K0 carbs from Dennis, (dragman) and another (better) set on EBay incorrectly advertised as Yamaha carbs, (?) a good K0 front fender from Tom, (El Cheapo) who thought it was for a K1, and was given a set of K0 forks and front caliper by a generous young feller here who's name escapes me, (sorry mate!) so I'm gradually finding what I need, but I doubt if I'll ever get a good original seat and airbox, so I'll probably have to go repro for them.
When it's finished, it'll be worth about $10,000.00 US here in Oz. If it was a "Sandcast" K0, I might be able to get 2 or 3 thousand dollars more, but as I'm not selling it, and there's no difference between the two to ride, that's hardly a concern for me. It's a K0, and arguably has the most "street cred" of all the SOHC4's.
So what is it really worth? I guess it depends on what you want to do with it. If you just want to on-sell it to make a buck, pay as little as possible for it and do the minimum work needed to get it into sale-able condition, but if you intend to restore it to ride and enjoy, then you should spend whatever you think you need to, to make it right for you. That's what I'm doing. Cheers, Terry.