pinning this explanation here, because the question keeps coming up.

"Drop rate depends entirely on the amount of ancillaries a culture group has, the Dark Elves for example have around 80 in vanilla (i think), then you have a bunch of randomly dropped ancillaries that are not culture locked (pidgeon plucker pendant). My mod adds a further 30 to the pool. But that doesn't count for multitude of transferable ancillaries that are also added to the pool under certain circumstances. Then on top of that you have mods like Rebanner and Drink It who also add numerous new ancillaries to each culture group.

So yeah hard to say, when you acquire a new item it's pulled completely randomly from the pool, so a common or a rare item have the exact same chance to be pulled, that chance being 1/X (X being the total number of ancillaries available).

That's the most concrete answer i can give sorry."