What's the economic value of having Twitter followers?
For example this Twitter account @ProfFeynman (Richard Feynman) has almost half a million followers. Do they make money off this? If so, how?
Followers = Potential Attention towards something. You can surely release something and point a lot of attention to a product or something with those followers.
Directly, no, they don't. However, it helps building credibility (to their target audience anyway) and credibility is always convertible into money.
That's the dynamic behind Instagram influencers, etc.
To Twitter or you?
You can get a direct economic value out of it by eg. promoting your products or services.
For someone like Richard Feynman, I would imagine the value lies elsewhere.