Deciding whether to keep a low-margin product that brings people in

One low-margin item accounts for a lot of first-time customer purchases. On its own it barely earns anything once packaging and staff time are counted. But people who buy it sometimes return for higher-margin products, so discontinuing it could be shortsighted. I'm thinking of tracking baskets and repeat purchases for a few weeks rather than judging the product in isolation. How have you measured whether a ‘door opener’ item actually brings profitable customers instead of just bargain hunters?