Should I negotiate the price or the monthly payment?

Negotiate the price, specifically the out-the-door price. Focusing on monthly payment makes it easy to lengthen the loan term or add products while the payment looks affordable, which can raise your total cost. Agree on the total price first, then compare financing terms separately. OffLot's "What can I afford?" tool works back from a monthly budget to a price range, so you can set your target total before talking payments.