Difficult decision as they don't often appear for sale in the condition you describe.
Is the BG to go into his machine, or on his wall ? Personally, if it is going in a frame - I would not sell it. It should be on a machine. Obviously, it is his machine, and it will increase its overall condition and value.
I think $250 is too cheap
I think $600 is way too expensive - and this MIGHT be it's worth on eBay
I'd say "mates rates" - $350 - It is a tough decision.
You could email Mayfair (who might have them) and ask for the value of the glass - I'd say US$250 would cover it - just for the glass.
