What is an Ambassadeur 5000 reel with serial number 107200 worth?

Depending on condition, between 20 and 200 bucks.

$20 for a beater that needs parts and fixing.

$200 for a mint one, with box, case, spare parts vial, tool and paperwork.

Plenty of those on eBay.

Do an eBay search and you'll get a pretty good idea what they sell for.