has it best on the 'at-the-gate-for-sale-upgrades'. Starts at $50, goes up based on length.
Not determined to be available within 3 hours before departure, and then I think, only for sale within 90 mins. of departure at the gate only.
The mileage-based upgrades, however, can not be used on the lowest fare classes (T for sure, maybe L and U).
's is similar, though the starting price is $35 (a better value I think, I've bought it a # of times).