Wow, both bikes gorgeous. Here in NZ the best CB750s , not sandcast, nor even K0 are hitting around $40K. Can't see why an equivalent condition 500 or 550 shouldn't be worth at least 1\2 that; they are rarer, cost near enough the same to restore and a lot of people, including contemporary road testers considered them better bikes.
Re the comments on the auction site, any Aussie members confirm good 550s are worth $AUD 20K? Also apply to 500s?
Last year I got $12K NZ for my 550 which was a pretty good bike; possibly could have held out for a bit more. Best I've heard is $15K for a mint 500. I'm picking they have gone up a bit since then.
I'm doing up a couple of 500s, but it's hard to say whether the extra money to be had from doing it up properly , would be paid back. I'm thinking the average one of mine would be worth maybe $8-9K at best, but would cost another $5-6K to bring it up to mint. Might not break even, let alone make it worth the extra work. Looks like this, minus the pipes. Any ideas?