I want to say that 'A is cheaper than B'.
Genki, the book, would formally suggest:
A のほうがBよりやすいです.
What would be a more colloquial way to say this?
Just remove the 「です」 which is used basically to make the sentence politer here (丁寧語), and you will have a more familiar version of what you want to express.
You can also remove the 「Bより」 and just say something like :
Aの方が(もっと)安い
You could also say "やすいのはAだ" or "やすいのはAです". Strictly this translates as "The cheap one is A". Nevertheless, if you have already been discussing the various merits of A and B, then this will give the impression of "the cheaper one is A".
You could even say "やすいのはA"