Fast food is typically cheap.
In Busuu, they make a distinction between günstig and billig. Günstig has a meaning of quality at an affordable price, whereas, billig means lower quality at a cheap price. Dictionaries list them as synonyms (and they are), however, they don’t convey the exact same meaning.
Yes, it’s just as you wrote.
For anyone who’s interested, and maybe this helps internalize the difference, here’s a little more background.
“günstig” literally translates to “favorable”, as it derives from “die Gunst” - the favor.
Die Gelegenheit ist günstig. - The opportunity is favorable. (= The timing is right.)
Das Spiel endete zu meinen Gunsten. - The game ended in my favor.
Er versuchte, meine Gunst zu erlangen. - He tried to gain my favor.
die Gunst der Stunde - the opportune moment
“billig” just means cheap.
Das war ein billiger Trick. - That was a cheap trick.