、。
Beer is taxed by malt content, so low-malt beer works out cheaper.
Literal
Beer [topic-は] malt [gen-の] content [depending-on-によって] is-taxed [because-から], happoshu [gen-の] side [subj-が] cheap [explanation-んだ] [emph-よ].
によって 'depending on / according to / by means of' is the formal variant of the instrumental/causal marker, here indicating the basis on which the tax varies. 発泡酒 is a specifically Japanese tax category — a near-beer drink engineered with low malt content to dodge the higher tax on proper beer. ~の方が is the comparative frame 'X side is more Y,' standard for two-way comparisons. ~んだよ explains and asserts.