Flights are cheap if you book them early.

Literal

Airplane [topic-は] early-discount [by-で] taking [conditional-ば] cheap [emphasis-よ].

早割でとれば安いよ — 早割 is short for 早期割引 ('early-bird discount'). とれば is the conditional of 取る ('to book/get'). よ asserts this as useful advice. Very casual and natural.