For example, I found seats on Aeromexico departing Los Angeles LAX on Friday, August 17, returning from Mexico City MEX on Tuesday, August 21.
You'll find other travel dates available in August and September. This fare is limited on United, but Delta and Aeromexico are widely available.
This is the classic fare on Aeromexico, which is only a few dollars more than the basic fare and worth the extra few bucks to be able to choose your seat, make changes (for a fee), etc.
You can get a slightly lower price by booking two separate one-way flights and booking your return flight in pesos. If you do this, make sure to use a card that does not charge foreign transaction fees to avoid any potential loss in savings.
Need a room in Mexico City? Here's what's available for the above dates on Hotelwatchdog.