Edit 26/05/20 - A couple of very useful tips from my fellow rally enthusiasts:
LaughinGuo says "I recommend grind money in 1 star rally event since u need 500 stage."
zX KiNGxKoNG Xz tested my shift button skillz and says "It also helps to save money if you fire your whole crew after winning all world championships. Your car will handle a little different but you will save a lot of your money."
Thank you KOSTAS A M GR!
Poops' original stream of conscious madness:
I found playing 4 star Extreme-Khana championships is the fastest way to make the money required to purchase all of the cars available in the game.
It's even faster if you are handed a four-event E-K with just the one race at Niger - if you place 1st in the other three events you can simply quit out of the Niger race and still win the championship by one or two points. Don't use 'quit activity', use 'quit race'! This also saves on repairs between races.
Not hating on that particular course, but it really gave me the shits. Apart from my utter failure to keep a RWD Mustang in a straight line on those vertical sand corners, the lap times are much more forgiving at the other events.
If no 4-star events pop up just enter and quit 1, 2 & 3 star events until a 4 star appears. This also drops the difficulty level for the rally events if you're having troubles with those.