I've always used "The Ultimate guide to Honda Motorcycles" by Doug Mitchel. It includes prices based on condition for all Honda bikes over 20 years old.
These were the values it gave for the 550 in 1975, for example: (from when Paul was asking what a reasonable price would be)
1) Excellent- "Restored to current maximum professional standards of quality in every area, or perfect original with components operating and appearing as new." -$2,000
2) Fine- "Well-restored or a combination of superior restoration and excellent original parts. Also, extremely well-maintained original motorcycle showing minimum wear." -$1,600
3) Very Good- "Complete operable original or older restoration. Also, a very good amateur restoration, all presentable and servicable. Plus, a combination of well-done restoration and good operable components or a partially restored motorcycle with all parts necessary to compete and/or valuable NOS parts." -$1,200
4) Good- "A drivable motorcycle needing no or only minor work to be functional. Also, a deteriorated restoration or a very poor amateur restoration. All components may need restoration to be 'excellent', but the motorcycle is mostly usable 'as-is'." -$900
5) Restorable- "Needs complete restoration. May or may not be running, but isn't weathered, wrecked or stripped to the point of being useful only for parts." -$600
6) Parts Bike- "May or may not be running, but is weathered, wrecked and/or stripped to the point of being primarily useful for parts." -$400
Taken from "The Ultimate Guide to Honda Motorcycles", by Doug Mitchel, 2005, kp books.