This paper presents the results of a survey and analysis of electricity tariffs and marginal electricity prices for commercial buildings. The tariff data come from a survey of 90 utilities and 250 tariffs for non-residential customers collected in 2004 as part of the Tariff Analysis Project at LBNL [2]. The goals of this analysis are to provide useful summary data on the marginal electricity prices commercial customers actually see, and insight into the factors that are most important in determining prices under different circumstances. We provide a new, empirically-based definition of several marginal prices: the effective marginal price, and energy-only and demand-only prices, and derive a simple formula that expresses the dependence of the effective marginal price on the marginal load factor. The latter is a variable that can be used to characterize the load impacts of a particular end-use or efficiency measure. We calculate all these prices for eleven regions within the continental U.S. The methodology developed here can be adapted to any particular customer or utility sub-sample that may be of interest.