So I've set up an example XP run AMS-BLQ. Outbound via CDG, return direct.
So that would be 15XP in Eco. At 290 EUR or thereabouts for an arbitrary date but probably doable for under 200. in Biz, 60 for 565.
Unfortunately, KLM does not give details in the CO2 options, only says whether you earn XP.
The four options and their presumed earnings go like this:
At 10 eur per XP, it's not super great value but good enough, considering you don't have to actually travel and spend time and accrue additional costs.
I will probably be taking advantage a couple times a year to give me buffer. If how the current situation is any indication, I need not be looking forward to a large amount of company travel, as I was 2.5 years ago. Consequently if I am to earn Gold, I am comparatively cash rich but time poor. Oftentimes, the answer would have been J class, but this is more flexible.