It really depends on condition. You can buy a "restored to museum quality" sandcast 750 for maybe $3500. However I probably wouldn't get $500 for my '78 as it is right now.
The more stock it is, the better, IMHO.
I had a cafe racer dream for a while, then just getting the damn thing to run after letting it sit for 2 years overwhelmed the project. Man do I hate those carbs.
Here's somebody who had a lot of fun with his:
http://community-2.webtv.net/@HH!D3!F9!A49DDFB60BA4/Roadster1928/CAFERACERHONDACB750/