The Amherst Student published an editorial some years ago saying that there were too few psych professors and too many physics professors at Amherst. Joe Rachiele and I wrote letters to the Student -- we meant to write a joint letter, but we screwed that up -- saying that it would be equally sensible to complain that there were too many psych majors and too few physics majors. It's very deja vu to see the Harvard Crimson rehashing the editorial, and Mankiw rebutting it, along very similar lines.
Mankiw's ideas for "pricing" the econ major are somewhat more detailed than anything we came up with, of course. The idea of requiring a minimum Econ 101 grade is a little distasteful -- it discriminates in favor of the best-prepared freshmen, at the expense of those who take a year or so to get up to speed -- and the multivariate calc requirement, though sensible, is econ-specific. But one could do other things -- e.g. move the first three Econ/psych courses to 9am and require attendance, turn down the heat in large lecture halls (thus simultaneously turning people off all large classes), have a mandatory field trip to Flint, Michigan (thus outsourcing the removal of econ majors to unemployed Michiganders), etc.