Disclaimer – I am not a film maker. Nor have I been associated with film making.
Here are my two cents. – I happen to be friend and colleague of a documentary film maker my response is based on my interaction with him.
Also, making money could mean two things a) Cover Costs b) Make some surplus money over your costs.
I use the term “money” interchangeably below. These points are over and above what Jithin has stated.
1. A lot of times they don’t make money.
2. A lot of times they don’t make docs to make money.
3. Grants are a pretty good way of funding your doc project. There could be several private, and state grants. PSBT in India is one. Like this one –. There are also theme specific grants like Climate Change etc, by international agencies. Yes, if you manage your budgets a bit you can make some money over the costs. But most of these grants are either very competitive or there are cliques which operate there.
4. The Problem with grants is that it sometimes is quite restrictive for the film-maker. It’s the same “funded research project” problem.
5. There are also Docs produced by Bollywood and Hollywood producers which are meant for a larger audiences, Ship of Theseus and An Inconvenient Truth being some which belong to that category. Again these producers do not necessarily try making money out of these projects.
6. Lastly, the economics of making a documentary is similar to any service/product which has an intrinsic value, however the masses/majority may not want to value it that way. For example – Take something like arts, specifically say “Madhubani Art” now if you leave it to the Madhubani artist to make a living of it, they day is not far when he/she would leave that profession for the lack of demand of that skill. In other words markets/masses would not want to pay-for/value that skill. One way to protect that art/skill is state/public/private philanthropic funding. The same logic applies to documentaries.
Hope these points add to the perspective of the readers.