I can say from experience that an Instagrammer with 100k followers is unlikely to get anything near $2700 to post a photo. This was probably true several years ago, but these days the number is closer to $1000.
The ROI used to be that good, and then the market became crowded and the consumers became more savvy.
Now the ROI is worse, the advertisers start lowballing, and the content creators keep accepting (because rent is due and nobody else is offering any higher, sooooo)
This is spot on. It's just like any other marketing tactic - it works well until everyone else starts doing it.
As for the pricing of posts, the lowballing is getting more and more dramatic, because there are many more "influencers" now. The only brands really shelling out are large corporations with large budgets looking for something very specific. Smaller brands can just reach out to a bunch of influencers offering only product in exchange for a post, and someone will say yes.
Basically the same way SV companies come up with their valuation. You ask someone to pay $1000, and if they say yes... that's the price. Now every company who asks the price is told $1000.
It's the agencies sitting in between brands and influencers who are that good. They're incredible salespeople, and since they get paid a percentage of the spend, it's in their interest to have as many influencers who can post, so they can sell as many posts as a brand can be talked into.