I have researched every US airport because fares are consistently high at Christmas. What I found is that I don't save any money by starting at a hub in Houston, Dallas, Phoenix, Denver, Salt Lake City, etc. - the fares are high no matter when I start so I am best just booking a nonstop or connection from SFO (LAX can sometimes be +/- $100). Even starting in MEX is fairly pricey on Aeromexico and you still have to get to Mexico City.

The only exception is MIA/FLL on JetBlue and American (maybe they have to fare match spirit) - so if you can get to south florida for free you might save some $$ to CUN.