That'll be to do with your sponsors and race goals. You make money by hitting your goals (which is said before you jump into a race)
And each sponsor will give out different amounts of money. The more tokens you get from a primary sponsor (such as wtf1 and iracing) the less money you get from them, but the less tokens you get from a primary sponsor, the more money you make. So look at the income and bonuses you get from each sponsor, I personally like to go in between and pick red bull as my primary sponsor.
Hope this helps a bit :)