Wanted to share a data point. There was one Mint seat open on a SFO->JFK flight. It was being sold for $1300 online and they wanted fare difference. At gate I heard someone ask for the cost to upgrade and it was $649, so they passed.

I go to board and they tell me that the flight is full but a crewmember plans to deadhead, so they're giving me a complimentary upgrade to Mint! Amazing, first OpUp I've ever had. I didn't even know it was possible on Jetblue. I guess being a single Mosaic traveler has it's perks