Pharmaceuticals manufacturers are gradually disclosing more information about how drugs are priced. That is a problem for drug supply-chain companies and their investors. Eli Lilly said the price it was paid for its Humalog insulin averaged $135 a patient a month in 2018, down slightly from the 2014 average. Over that same period, however, the average list price—the sticker price before rebates and discounts are considered—rose more than 50%.