In a recent story match, I had 65 power on the board, and the Ghalta in my hand still was requiring me to spend 9 mana to cast after fetching it with Growing Rites of Itlomoc.
There were no edfects in play imcreasing its castong cost.
FWIW, I’m not seeing this issue. Playing a game right now with a single Ghalta in play as my only creature, and the Ghalta in my had is reduced to 12 mana as its cost.
I’m wondering if it’s specifically when Ghalta is fetched maybe? I’ve now started to see it working with other methods of getting it onto the field. Or perhaps the encounter was bugged.