Here I'll go over some of the main ways that I find low-cost flights primarily oriented towards cash purchases.
Google Flights
Google Flights should be your baseline for low-cost flights. I've created a separate article for it here.
Flight Connections: See Potential Flight Options by Airline Routes
Baggage fees
Distant Airports & Positioning Runs (Especially for International & Points Based Flights)
Travel Hacker Following
Looking directly on airline websites for miles deals or $ deals including promo codes:
Signing up for SPAM directly with airlines & others
Flexibility
Carrier Rankings
Seat quality within the same carrier
Airports
Lounge options and quality on itineraries
The Value of Time
Additional credit card savings
Caveats