Ambassador, British Airways; FlyerTalk Posting Legend
Join Date: Apr 2012
Location: Leeds, UK
Programs: BA GGL/CCR, GfL, HH Diamond
Posts: 42,967
Would this be a cash WT+ booking? Basically it is the same calculation as before, the difference between taxes/fees/charges for the two cabins. You should be able to calculate this by pricing up the flights you want as a cash fares on ba.com and looking the two price breakdowns.
As for the avios cost, well that is a bit harder to try and work out.