Average upgrade price at check in

Can anyone tell me what the average upgrade price is at check in from economy to premium.
Travelling back on VS72 (744) most likely on a non LGW upgraded aircraft as it will be on 24th June. I know from last year there was plenty of space in Premium and Upper, with upper only having 1 person on board!!!
I know it may depend on the day but was wondering if folk had some idea on rough costs for a one way upgrade. I have checked my fare and it is not in an upgradable bucket to use miles (shame as we have more than enough) and the cost to upgrade to bucket is £600!!!!!
Cheers in advance.
Travelling back on VS72 (744) most likely on a non LGW upgraded aircraft as it will be on 24th June. I know from last year there was plenty of space in Premium and Upper, with upper only having 1 person on board!!!
I know it may depend on the day but was wondering if folk had some idea on rough costs for a one way upgrade. I have checked my fare and it is not in an upgradable bucket to use miles (shame as we have more than enough) and the cost to upgrade to bucket is £600!!!!!
Cheers in advance.