This is the 8574th most frequent Swedish word.
billigt
Cheap.
Here, 'billig' is used as an adjective to describe the noun 'bok' (book). It means inexpensive in this context.
Jag köpte en billig bok.
I bought a cheap book.
In this sentence, 'billiga' is the plural form of 'billig' used to describe 'semesterresor' (vacation trips). It indicates that the trips are economically affordable.
Semesterresor kan vara billiga om du bokar tidigt.
Vacation trips can be cheap if you book early.
'Billiga' is used to describe the noun 'lunch', showing affordability and good value for the food provided at the lunch time.