Someone else told me it's not supposed to be random each time, so maybe it is a bug, but maybe it isn't.
Brewing potions on my mimic is wildly chaotic. Serious healing, for instance, can range from as little as 25 gold to nearly 60. It changes with each casting, with different batches brewed in the same brewing potion costing wildly different amounts. It's more stable when casting on the empty bottles themselves, and most of the seeming bugginess emerges when using a potion cauldron. This is not limited to only serious healing but all potions.
Maybe it's not a bug, but I figured I'd report it in case it is.
No bug. Working as intended.