Newby here so hope I'm doing this right...

I've used this feature on 4 different bookings since December.

1 - LHR-DXB W-J = £220.00 7 days out of travelling

I had checked this 2 days earlier and the price was £260.00. Unfortunately (or as it worked out fortunately) for me, by the time I verified with lady purse strings who was also travelling that it was a good deal, the offer had gone. When I saw them a few days later £40.00 cheaper I snapped them up.

This leads me to think there isn't a set figure. Desperate times call for desperate measures and depending upon how much BA need your seat dependant upon overbookings (whichever class you've booked) I think could dictate what upgrade price you are offered.

Also I've noticed that I could then upgrade further from J to F for £200.00

The flight was operating with a 100% pax load on the day which lead me to think perhaps this was a way for BA to generate additional revenue on what would otherwise be complimetary OPUP's?

Since then I've upgraded on the following:

2 - LHR-FRA M-C = £69.00 14 days out of travelling
3 - FRA-LHR M-C = £69.00 19 days out of travelling
4 - VIE-LHR M-C = £79.00 6 days out of travelling

5 - LGW-BGI W-J = £399.00 14 days out from travelling

I wouldn't normally have said £399 was great value (it's in line with the fare diffrence between I and T classes when we booked our T class tickets), however I managed to bag us an avios upgrade on the inbound for £71.00 + 12500 points per person so this works out to be the equivilent of £235.00 each way.... admitedly we are still only earning W tier status on the inbound.

Incidentally for anyone thinking of doing this, it's important to complete the proactive upgrade transaction first before you make any upgrades using avios points. By doing so ba.com removes the ability to make any further upgrade changes. I had wanted to upgrade the proactive upgrade from J-F as I had seen this prior to securing the inbound leg on points for £200.00. I stupidly completed the inbound avios upgrade then went back to make the final adjustment to the outbound leg and the function had disappeared for this booking. I guess you live and learn