The gold bar method has been patched
so the easiest way to earn money is to follow the glitch in this video.
(Thanks to Reborn Insanity
and others for letting me know).
As far as I know, all decent glitches and exploits have been patched, so the easiest way to earn money is via collecting gold bars (thousands of videos on YouTube show these - but many contain spoilers, so I won't link them), or main missions. I would say use the main mission for this, the money is not a lot once you get deep into the story. It is recommended to collect the gold bars in the epilogue.
Once you have enough money to fully upgrade the camp, do so, especially so you can fast travel. Donating just $250 will unlock:
When you have loads of money, head to Saint Denis' tailor. You should be able to fast travel here if you have previously been here.
Make a new save before
you enter. When you enter, you should get pickpocketed. Chase the thief, hogtie him and then loot him. If he took more than $250, you will get that money plus a bit extra. He took $490 from me and I gained $610. The reason for saving is that you don't always get the money back. I tested it on three different save files so I believe this event happens for everyone. Please let me know if that is not the case.
You can then spend $5000 for: