If you walk, it won't cost any money. That's the cheapest way to get there.
Walking is free. Why don't you do that?
ーIf you walk, it won't cost any money. That's the cheapest way to get there.
「歩いたらお金はかからないよ。そこに行くのに一番安い方法だよ」
to cost で「お金がかかる」
the cheapest way で「一番安い方法」
ーWalking is free. Why don't you do that?
「歩くのはタダだよ。歩いたら?」
free で「無料の」
Why don't you ...? で「…してはどうですか?」
ご参考まで!