According to this, Facebook revenue per-user-per-year is about $30, so $2/mo (post-Apple cut) is probably not far off for Twitter: https://www.statista.com/statistics/234056/facebooks-average.... But of course, why replace that revenue when you could double it?
edit: here's an ARPU estimate for Twitter in 2016, it was around $2/quarter: https://www.statista.com/statistics/430874/twitter-annualize...