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.