I'm using a PC running windows 7 ( english os ) and the pillars client is in Italian: the vendor having issues is Igrun in Copperlane (the other vendors in the same location are fine). Switching the game client to English made the prices to change correctly, but changing back to Italian caused the prices to be wrong again and really high.
I've tried starting over with a new character and the issue happened at the Black Hound Inn in Gilded Vale and the other vendors in the same area were fine.
Same issue with both Steam in Italian and English.
I never noticed this issue before the last patch if this can be helpful.
ps. Thanks for the updates!