I use a Fluval E-Series 50 watt heater.
I used to have a Fluval M-Series, but my betta would see a mild reflection on the chrome like coloration in the glass of it and fight it day end and day out! That's why I got the E. It also has a thermostat that lights up which was nice.
77.4 F is not bad at all for a betta. I like to keep my bettas at 78-80F. They seem to enjoy the heat though.
My recommendation might not be all that cheap though. I really only use cheap heaters in my temporary QT tanks. I've never seen these at Petco - I only ever see their brand, tetra, or aqueon. I use the smallest aqueon in my bookshelf tank (6.6G).