RTOs now cover well over one-half of customers and sales of electricity in the U.S. As they have expanded in geographic coverage and functional scope, controversies have arisen about their rapidly growing costs and overall effectiveness. We model their costs, exploiting the fact that the seven existing RTOs initiated their various functions at different points in time over their roughly ten-year history. Specifically, we investigate the costs of each of the market functions administered by RTOs, the relative costliness of different RTOs in petforming the same functions, the possibility of learning economies from either their individual or industry-wide experience, and economies of scale over the sampled range of RTO "output." Our results confirm the importance of some of these factors but raise doubts about others, in all cases based on systematic modeling and data analysis. doi : 10.5547/ISSN0195-6574-EJ-Vol32-No4-7