Publication fee for reply to comment? I have recently published an article in a "renowned" open access journal which cost us 2000 USD. I say 'renowned' - not to brag, but to highlight that the journal is not the kind of new age OA journal from which I would expect questionable practices. 

Another group of researchers now started the process of submitting a comment to our article, to which we crafted a reply. After review, the journal now wants to charge them, as well as us another 2000 USD each for the publication of the comment/reply pair. This comes as an absolute surprise to me.

In my opinion, this practice is actively discouraging scientific exchange, and it seems simply excessive to ask for another 4000 USD. Furthermore, should the reply not be considered part of the comment, therefore, if anyone, the commenter should pay for the publication cost?

I am wondering if anyone has heard of something like this or has experience with publishing comments/replies in Open Access journals?
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.