Dont have all the answers but a real life example of a locally targeted social ad campaign for a big 2 letter brand that starts with "3" that targets the automotive aftermarket crowd in a 2.5M city has a monthly budget of 5-15k. That's a very niche market. The consumer products division is much larger. The medical devices division is even larger.
You get the idea. The deal are high and that's just ad spend budgets. Design. Creatives. Social media management. Production. Really brings the prices up.
A social media management service that takes care of all the responses and post generation (images, design, etc) across Instagram, FB, etc can easily charge 3-10k/month.
A small business for example a successful crossfit GYM pays $400-600/month. But a larger business commonly pays 1.5K/month just for social media management and some social post productions (photos and edits) and maybe some ad campaign management.