Here is a video of some old fashioned grinding.
It show cases 3 missions that are available at rank 5, 35 and 45.
These do not require any special cars, helicopters, planes, etc.
All you need is your character and any gun...
It is best to time the missions so that you complete them just over 4 minutes in mission (time it from when the banner drops at the start of the mission to when you deliver the car or docs).
If you run into game variations or issues, the time for payouts to change is 4+, 6+, or 8+ (1, 2, 4, 6, 8, 10, 12, 15 are the times where payouts go up). 100% pay is at 4 minutes and it increases 20% every two minutes. So you are going to make more money doing the missions at just over 4 minutes than waiting for the 15+ mark.