Which credit card feature determines the cost to use one credit card to pay off money on a different credit card?

The feature that determines the cost of using one credit card to pay off another is the balance transfer fee. This fee is typically a percentage of the amount being transferred, often around 3% to 5%, and can also include interest rates that apply to the transferred balance. Additionally, the annual percentage rate (APR) on the new card can impact the overall cost if the balance is not paid off within any promotional period.