Actually, i currently have one here but it's not mine.
It's always extremely hard to put prices on these old beauties. To some people is just 25 Kg's of old junk while it may have great value to others.
These are great machines and a well serviced machine like this will certainly be worth a bit. It's like with all vintage B&O's; Condition is everything.
The sound quality of the B&O wirerecorders generally is unbelievably good and the Beocord 506K is also a record player.
Maybe you should get in touch with Roland in Germany. He restores and collects all the B&O recorders and may be able to help you. His homepage is definitely worth a visit:
http://www.makarateyp.com/RZBO.htm
I will attach a photo to tell what it is we're talking about.
Martin