Relative cross-border retail prices, in a common currency, comoves closely with the nominal exchange rate. Using a data set with product level retail prices and wholesale costs for a large grocery chain operating in the U.S. and Canada, we decompose this variation into relative wholesale costs and relative markup components. We find that the correlation of the nominal exchange rate with the real exchange rate is mainly driven by changes in relative wholesale costs, arguably the most tradable component of a retailer's costs. We then measure the extent to which national borders impose additional costs that segment markets across countries. We show that retail prices respond to changes in wholesale costs in neighboring stores within the same country but not to changes in wholesale costs in a neighboring store located across the border. In addition, we find a median discontinuous change in retail and wholesale prices of 24 percent at the international border. By contrast, the median discontinuity is 0 percent for state and provincial boundaries. * We are grateful to