I think it's somewhere around 15 Steem for a post before it gets linear but I'm not sure. Why would anyone upvote comments in HF21, a single upvote will be worth a lot less for what I understand which is sub-optimal as you will have to drain more voting power to get above the dust treshold and it gives less curation compared to upvoting a post that has a lot of upvotes.
RE: HardFork 21 | Gameplan & Predictions