Publication fee for reply to comment?
I think comments papers are an important (and sadly neglected) part of post-review quality control in academic publication. The fact that someone wants to write a comment paper is suggestive of a problem with the peer review of the paper in question, if only in that it left substantial questions unanswered (or unasked).  As such the journal should not charge either party for the comment paper or the response, if only to show that the journal takes quality control seriously, even if they do make all their money from page fees, I think it is in their long term interest.

Writing comments papers should be incentivized, not discouraged by financial hurdles.