A loss leader only works if enough of the customers it attracts come back and buy something profitable later. Without that repeat behavior, it's just a discount that loses money.
Before running one, know this number
Your repeat purchase rate within 90 days. If it's low, a loss leader is unlikely to pay for itself. If it's healthy, a loss leader can be one of the cheapest ways to acquire a first-time customer.
The simplest version to test
A single, well-known product priced at or near cost, paired with a genuine call to action toward a higher-margin companion product at checkout.