I went in the direction of something that needed freshening but not major work. You're talking more than $500 for one in that category that's also "collectible". The CB400F is definately in that collectible category. I paid $1,800 and it needed a tune up and new tires. I saw one locally that needed more work in the $1,200 range. There is generally good parts availability and support. I'd think the mid 70's 550 and 750 would fall into the same category. I see ads for quite a few that have may have low mileage but have been sitting.
Here is a local ad. I talked to the guy and may pick this up if it's in as good a shape as he says and is still available although I have enough projects on my hands in the form of cars. If you're talking about something that needs more work I would think $500 is a good range. I'm no expert but common sense would say a top end on this would be simpler than a 911 or 914.
http://seattle.craigslist.org/sno/mcy/762084466.html
Mine - 1975 CB400F
