Should-Read: Robert C. Feenstra and David E. Weinstein: Globalization, Markups, and US Welfare: “We work with symmetric translog preferences… http://www.journals.uchicago.edu/doi/abs/10.1086/692695

…find that between 1992 and 2005, US import shares rose and US firms exited, leading to an implied fall in markups, while variety went up because of imports. US welfare rose by nearly 1 percent as a result of these changes, with product variety contributing one-half of that total and declining markups the other half…