I would have expected any price difference to have something to do with the cost of making pills. If that's not the case... is there a competitive market in powder but not a competitive market in pills, or do all the sellers agree on this same method of price discrimination, or what?

I'm inclined to agree. A large difference between the price of pills and the price of the powder tells us a whole lot more about the depth of the market than about taste. If the market for piracetam were large one would far more closely track the other.

There's usually a cost to the convenience of pills, yes; but the greatest the difference, the more convenience is being provided (because otherwise people would just cap their own or not buy the substance at all). Piracetam seems to have unusual differentials, pointing to some greater convenience being provided - which I believe to be related to its revolting taste.