Since the introduction of RFS for long haul flights there are anomolies here. Basically BA trying to have their cake and eat it. Agents are clueless when the prices BA IT throws out for upgrades is often incorrect.

The BA website states that the difference in Avios and TFC between the original and upgraded classes is due. However, often they ask for the Avios difference between the RFS amounts yet the TFC difference between full cash ticket TFC amounts. This results in a higher Avios charge as the increased RFS Avios amount is supposed to partly compensate the increased TFC.

It gets interesting sometimes when upgrading to F, for which there is no RFS. If you are lucky, this will result in a very low Avios upgrade price.