Originally Posted by MSPeconomist

Traditionally DL paid a $200 voucher plus the fare difference when customers were involuntarily downgraded.

Was this a (DL) award ticket or did you pay (DL redeemable) miles to confirm FC upgrades? If it's the latter, your fare difference would be the points you paid, attributed to the two segments proportional to mileage. For a FC award ticket, DL should be able to check what the same flights in coach would have cost in miles at the time you obtained your award ticket.