Im guessing the price tag on these is about 10m+
That is way too much, even if it is right that the charm actually gives a 100% bonus. 1 mill I could understand, but more than that doesn't make much sense IMO. 1 mill equals 250 super xp lixes at 4 k apiece, that will give you twice the xp of the charm (if it is 100% and not 20). That is 60 hours of lvling followed by 60 hours of boss hunting time, which will probably give you the 1 mill right back, for the same xp amount that 120 hours of lvlgrinding with the charm would give.
Dont get me wrong, i think the xp charm is dead cool, but its main benefit is for lower lvls who wouldnt use lixes for lvling, or for random gameplay xp besides lvling for higher lvls. That random xp is rather limited, and for lvl grinding a higher lvl will most likely use lixes anyway.
Given the misc charm drops in this summer event, that will bring back long awaited competition for the misc slot, I do not believe that the xp charm will reach insane prices far above 3 mill. (at todays "nominal" value that is, of course things could change if new top mounts or other lux gear in a later update was launched at 5 mill prices etc)
There are (luckily) players who play CH to play rather than to grind, and to those the xp charm would be very nice. But these players are probably exactly the players who will not bother to pay insane amounts for xp gains.