There are 2 versions for B series machines, and both are somewhat scarce, or at least, desireable (manual lift and cable-lift). There are lots out there, but limited #'s for sale. A great condition sicklebar, can bring $400. The manual lift version will be trumped by a cable lift type (which ties into the tractor's lift system), but a sickle bar shopper will take either type, most likely.
I see them frequently sell (resonably, as there are always 'steals' that aren't the norm) anywhere from $200-350. If it is a real clean, complete, cable lift variety, with helper spring, pulley & mount for the frame of the tractor, support & transport chains, swath board and rear mount bracket, I'd say list it for $450, and see where things go. Any missing parts, wear or damage will quickly drop the price from there. Personally, I'd never let one go for under $300, if in operating condition.