It’s not worth it. How much gas do they waste idling in line at the border crossing waiting to get back in? San Ysidro and Otay Mesa don’t exactly have short wait times.
you've got it amigo. Also factor in distance. Conservative estimates say that every mile you drive costs fifty cents. I guess if the gas is just a few miles away, and you don't care about the quality the Mex gas may be worth it. Otherwise people driving more than ten or twenty miles are losing money with added depreciation plus wear and tear.