For what it's worth, I found (and booked) a RT JFK-LHR trip in late February where each leg was 2 certs. (Plus tax on the LHR-JFK leg.) Effective price of the upgrade from Blue wound up being around $400 per cert. Also found a JFK-LAX flight in mid-April with a 1 cert. upgrade option, in which the effective price of the upgrade from Blue was a little higher, around $500.