Originally Posted by Sjondorn

From New policy for paid and miles upgrades (AF/KL) - sept 2017 :
SFO->CDG: $2700
HND->CDG: $3800
CDG->HND: $2350

Mileage upgrades are exorbitant pricing regardless of the class, on most data points. But two times I saw a cheap upgrade possibility that can be done through booking, recent booking I made in P has price difference about $1200 between J and P (SFO-CDG-PRG) one way (booked in advance), so if you want to get your way to P either book it with miles directly or scout some cheap "booking time" price difference.