At the beginning of Red Dead Redemption 2, Arthur Morgan and the Van der Linde gang are fairly poor. However things improve as soon as you can start exploring the world, as there are countless ways to make money from activities such as committing robberies, hunting bounties, selling parts harvested from animals and so on.
The situation is quite different in Red Dead Online though. Players start poor and can remain poor for quite a long time, since there aren't many opportunities to make money, at least for now. There is one way, however, to earn money quickly in the early game, making it easier to purchase good weapons and any other type of item.
Unfortunately, this quick money making method has the potential to ruin your Red Dead Online story experience, as you will be playing Story missions that you may not have unlocked yet. If you don't want this from happening, just try to get through as many as possible before attempting to earn money through the method described below.
Additionally, you won't be able to use it once you have progressed past a certain point, so think long and hard on how to invest the earned money.
To start making money quickly in Red Dead Online, you need to unlock the ability to Free Roam. Once this is done, you can open the Free Roam menu by pressing left on the directional pad. Select Quick Join from the menu and then Story Mission On-Call. By doing this, you will be joining an on-going Story Mission playthrough: the later in the story the mission is (even ahead of where you are up to personally), the better the rewards will be.
Unfortunately, there is no way to know exactly how much money you can make in a set amount of time by using this method, as some missions are more rewarding than others. There are a few ways to maximize profits, however: make sure to complete each mission without dying, as restarting from a checkpoint reduces the final reward for the mission, and avoid playing the same mission more than once, as you will only be rewarded with XP for repeated playthroughs.
Investing the money you obtain in an optimal way will also help you earn more money once you have completed several Story Missions through this method. Getting a powerful rifle, like the Bolt-Action Rifle, is a good way to go, as you will be able to get Perfect Pelts from big animals, allowing you to get a good amount of money for each one sold.
There are a number of other ways to earn some cash early on in your progress. Once you have access to Free Roam and Hunting, Fishing becomes unlocked as well, which is another great money-making activity. It's something you need to grind, but can bring in quite a bit of income once you start catching large fish. Fishing Challenges boost this activity's earning potential as well.
Looting discipline is another thing to keep in mind. You may not think that looting every tiny knickknack is worth the effort, but they add up in the long run, and selling them off in bulk can give you unexpected cash boosts. Just make sure you loot absolutely everything you come across!
Multiplayer Playlists are another way to earn some cash quickly. These playlists string a number of multiplayer matches together within a given game mode, or as a mix of multiple depending on the category. This just might be the least grindy method too, giving you more gameplay variety than just farming fish or pelts.