May be I am out of touch with current situation, but paying 15 dollars for a meal in McDonalds in US seems to be a bit of an overstatement. My understanding is that fast food prices are about 1:1. (You pay 4 bucks in US, 12 pesos here). Some prices are lower because of local specifics (beef, public transportation, health insurance etc), but in general they are comparable with prices in the States, assuming you don't live in Manhatten.