First of all these are the REAL NUMBERS, not a estimate.
It takes 2800 points to reach mastermind.
If you play a ranked fragile alliance and get a good ammount of money and win you are looking at anywhere from 50 to 70 points.
If you play arcade bridge level and get out with 4 million on level one its 25
If you play airport and run up the steps for 10 rounds without looting its 170 points. Takes about 15 minutes. This seems to be the best and fastest way to level up and I'm pretty sure if you go past round 10 you get even more points per round.
In order to do this start the airport level and run into the open gate area near the main hanger, you will see a set of steps, run up and open the door. Enter and run to the right catwalk and open the first door, run up the next set of steps and get in the chopper. While you go try to be a traitor for a few more points I wouldn't bother as you need to let someone loot first which takes way too much time.
The traitor method may earn a few more points but consider the time it would take to do it and the chance of getting killed. It's not worth it and it's over all faster to just hop in that chopper.
If you can't understand the tactic you can see a video on the airport tactic, please read the guide here for the "the love of money" achievement.
Good luck, use my airport method and you will be a mastermind in no time.