I had a pimp once walk into the bike shop I worked in to buy a tricked out pink pinarello with all super record components. He paid I think it was $1500 for the bike (those were the days) in 20s with one girl on each arm. I actually tried to talk him out of buying such an expensive racing bike but he knew exactly what he wanted. He wanted the best bike we sold as long as it was in pink.
I initially thought the Motobecane mixte was worth between $200 and $300. But given the location, the backstory of the bike, and its condition, I think north of $300 may well be reasonable. This is a classic vintage bike and it looks pretty darn new.