BS”D
HUH? Potato starch sold as cream of tartar? That’s a federal offence in the US unless the bottle is clearly labeled as a cream of tartar subsitute. Pls recheck the bottle.
Maybe the potato starch is added to keep it from coagulating and it is davka used so that the cream of tartar is kasher lepesach.
Yes it is expensive – I do remember paying $3 or so for a small box of OU certified cream of tartar years ago.