Don't make it a deposit. Just charge people to do it.

Metafilter has done this for years quite successfully.

Making people pay to comment removes certain types of low quality and spam comments, but I believe it can actually worsen polarization. Most people with moderate points of view aren't going to be nearly as motivated to comment as those who have extreme views, and therefore will be even less likely to do so with more barriers in place.

I agree with this, but doubt that a refundable deposit vs. a nonrefundable entrance fee changes this calculus significantly.

making comments subscriber-only would at least alleviate the "paywall" issue.