The trade
Reranking adds a per-query fee and removes generation tokens, because you send four good chunks instead of twelve mediocre ones. Whether that nets out positive depends on your chunk size and your generation price.
At typical numbers it does, because generation input costs far more per token than reranking costs per query. The calculation is worth doing rather than assuming in either direction.
The stronger argument
Even where reranking loses on cost, it usually wins on quality. Irrelevant context measurably degrades answers, so cutting from twelve chunks to four often improves output at the same time as reducing input.
If the numbers here come out negative, justify the reranker on accuracy and treat the fee as a quality cost. Do not justify it on a saving that is not there.