Cheap airfare and Economy class

Economy class and cheap airfare are semantically related In some cases you can replace phrase "Economy class" with "Cheap airfare", this phrases are similar.

Economy class noun - A class of accommodations on a ship or train or plane that are less expensive than first class accommodations.

Cheap airfare and economy class are semantically related in inexpensive class of travel topic. You can use "Cheap airfare" instead a noun phrase "Economy class".