If it's anything more than $1 a month, I won't pay for it. I think most users would be fine with a few dollars one-time fee or something tiny like $1 or less/month. If it's to curb the bot problem and there really are tons of bots, then any charge will be a problem for those operating bot networks.
But I think most users simply read and not reply anyways, so this may not affect a lot of people. I for one have only 500+ posts after signing up ~15 years ago. I primarily started using it to stay up to date one some things anyways.