5 clients, 12 posts a month
5 clients × 12 posts × 1.5 hours per post = 90 hours. At 75 USD an hour that is 6,750 in labor, so a starting retainer range of roughly 8,100 to 10,125 a month (20% to 50% added for overhead and profit) works out to about 1,620 to 2,025 per client.