In markets where spatial competition is important, many models predict that average prices are lower in denser markets (i.e., those with more producers per unit area). Homogeneous-producer models attribute this effect solely to lower optimal markups. However, when producers instead differ in their production costs, a second mechanism also acts to lower equilibrium prices: competition-driven selection on costs. Consumers’ greater substitution possibilities in denser markets make it more difficult for high-cost firms to profitably operate, truncating the equilibrium cost (and price) distributions from above. This selection process can be empirically distinguished from the homogenous-producer case because it implies that not only do average prices fall as density rises, but that upper-bound prices and price dispersion should also decline as well. I find empirical support for this process using a rich set of price data from U.S. ready-mixed concrete plants. Features of the industry offer an arguably exogenous source of producer density variation with which to identify these effects. I also show that the findings do not simply result from lower factor prices in dense markets, but rather because dense-market producers are low-cost because they are more efficient.