content: Two factors. Losses and congestion. Every mile away from the source you send electricity the more I^2R losses you have, and if you're importing a lot of energy into a city from the surrounding areas, you end up overloading the transmission lines going into the city the closer you get to the local distribution networks.  So it's not usually a negative marginal price for power, it's just that the price a Transmission Owner or Independent System Operator is willing to pay will result in a negative price after fuel costs and transmission prices are factored in. No one is going to pay you to use electricity.