I wouldn't say Interjet is a "no frills" airline as it has free food and drink with very comfortable new planes. I would say it is on a par with an airline like BA and way ahead of any U.S. airline. Their prices start from 190 USD return.

Vivaaerobus used to fly to Oaxaca but have stopped (they were very cheap but very unreliable). Aeromexico fly that route but are way more expensive (but do give you a lot more options). Many people go by first-class bus - 6 hours for 35 USD each way.