My first thought is that pricing should based on the number of followers;

0 - 999= $9/mo 1k-10k = $29/mo 10-20k = $39/mo 21-30k = $49/mo 31k-50k = $59/mo 51k-100k = $79/mo 100.01k-300k = $129/mo 300.01k-1mm = $259/mo