Medicare’s overtime deliberations on whether to cover an expensive gene therapy is serving as a valuable dry run for the coming launch of dozens of costly breakthrough treatments.

The Centers for Medicare & Medicaid Services will soon decide whether to cover CAR T-cell therapy, a novel treatment for leukemia and lymphoma that can cost nearly $1 million per patient. It involves extracting and then genetically engineering a patient’s immune system T-cells in a laboratory, multiplying the modified cells, and then injecting them back into the body to attack cancerous cells.

The sticker shock is a jarring wake-up call for...