During the transition of the electricity market from planned to market-based, industrial and commercial (I&C) electricity users have gradually shifted from planned electricity prices to market-based pricing. To promote this shift, an electricity purchasing mechanism via agents has been established in China. I&C users who have not directly participated in the electricity purchase market buy electricity through power grid companies, facilitating the implementation of the market-based electricity pricing policy. Firstly, the price formation mechanism of electricity purchased via power grid companies is analyzed, and a price model of electricity purchased via agents is established. To address two issues caused by the differences in electricity purchasing structure, namely the price gap between agents and market-based users and the large seasonal price fluctuations of electricity purchased via agents, an optimization method for electricity purchasing prices based on planned-to-market electricity allocation is proposed. Next, the relationship between planned-to-market electricity supply and demand within the planning cycle is studied. To minimize the price differences between I&C market users and agents, the proportion of wind and solar power generation allocated to the planned and market electricity, along with the monthly distribution ratio of purchased electricity, is optimized, with price fluctuations of electricity purchased via agents acting as a constraint. This helps dynamically adjust the price of electricity purchased via agents. An analysis of the annual source and load data of a province in China shows that the proposed optimization method for electricity price can effectively reduce the price differences between two types of users and minimize the price fluctuations of electricity purchased via agents, facilitating a smooth operation of electricity purchasing via power grid companies and supporting the planned-to-market pricing transition for I&C users.