The low end of the range a good deal. I expected the high end to be closer to the difference, but still less than if you just paid for the upgrade. However, the math for my Alaska cruise doesn't bear out that assumption.
For my Alaska cruise, the range to bid on Q3, Q4 from PG is $715 to $2285 per person, so $1430 to $4570 per stateroom.
To book that cruise now in PG, Sailing Soon is $5,118 and Cunard fare is $7,998 (per stateroom)
To book that cruise now in Q3, Q4 Sailing soon is $7,998 and Cunard fare is $10,878
But with the way prices change, your starting point in PG could be more or less than the fares currently available. All you can do is try some dummy bookings to see what the going rate is and decide if a bid is worth it.