Prescription Drug Rebate Reform Act of 2019 This bill requires private health insurance plans to set any coinsurance rate for a covered prescription drug based on the net price of the drug (the price after accounting for rebates and discounts provided by the drug manufacturer) and not the list price (the initial wholesale price set by the drug manufacturer). This requirement applies when a plan holder has an unmet deductible or other coinsurance obligation for prescription drug benefits under the plan. The bill does not prohibit a plan from requiring a copayment provided that such copayment is not based on the cost of the drug.