This is my first solution, so be nice.
It's a simple achievement that really does not need a solution.
I just wanted to let you know that this is a cumulative achievement.
I spent a couple of M to buy coaches/players. So you don't need to have 10 milion at once.
It unlocked for me when I was in Serie B (Italian Second Division) and I had about 3 milion. So don't save your money and just spend it to make your team better or to engage coaches.
TanukiV said in the comments that it unlocked for him during the fourth season when he had 9.3 M and definitely spent more milions in coaches and players. So the achievement it's probably bugged or has a strange way of counting the money.
It also seems from various comments (Banco Billy and naoya0831) that prize money does not add up to the count.
Thanks to all people that commented the solution and helped.