One tip to increase $$ and XP is to let the counter run down on side missions and only finish when the clock is ticking ie in the final 10 seconds. This will generally double XP and give 2-3 times the cash, up to about $20, good few nuggets too.
There is a downside though as you often have to sit at your destination for about 10 minutes, idling, not earning, staying alert enough to watch the map for pink blips, (countering current if you're in a boat) ready for a quick trot over the line. Miss it (I have dozed off!) and you've wasted a lot of time for nothing. I'm not even sure the balance is worth it, for the empty time, and it's hardly what I switch RDR2 on for, to sit on a wagon for 10 minutes with the reins in my hand and the guy on the duckboard next to me saying "Why are you doing this to us..?"
I wondered too...