We have recently returned and upgraded to business. We went LGW to BKK via DXB. We upgraded the LGW DXB leg when checking in online via a promotion offer (couldn't upgrade any other way as tickets were booked via 3rd party company) for 3 of us it was £1300. Great experience, could not fault it at all.
On our return we upgraded the BKK DXB leg of the flight, it costs us £2300 for the 3 of us on a shorter flight than the one we upgraded previously. On this occasion, although good, it was not as good an experience as the first time. The A380 this time was older, not as comfy, the lay flat bed did not lay flat completely as the previous one did.
So, the airport upgrade cost us more, for the shorter flight and was not as good an experience as our first upgrade.