HUGE difference in price. The engine cases have a rough texture to them because they were sandcast. Unlikely you will find one of these anyway.
Values vary as much as condition. Look to spend anywhere betwen $500 for a project and $5000 for a pristine example.
These bikes will always be desireable to a point. Pretty much the first superbike and they made a lot of them so there is quite the cult following.
Parts are very abundant and pretty cheap. Tons of parts on ebay. Join the sohc4.net forum for the largest wealth of info on these bike available anywhere. Lots of parts and bikes for sale there too.
I recently built a cafe from a cb550. Heres what I started with:
..and heres what I ended up with:
You can go here to see some pictures of the progress:
CB550 Cafe-----"Retro Documentation" for Instant Gratification