As you probably know their are two castles in the game, one costing 15 mil (Edinburgh Castle) and the other costing 10 mil (Bamburgh Castle).
A tip on making money, other than selling cars in the auction house, or just endlessly playing the game is to take advantage of credit payouts in specific cars skill tree.
Depending on what version you have or if youve got the DLC, All 7 Formula Drift cars are free in the auto show and also have a 300k payout that takes 19 skill points to reach per car, so if you’ve got 133 skill points lying around, there’s 2.1 mil right there to grab. If they are not free for you then The spreadsheet below contains a list of all cars you can buy that have credit rewards in their skill tree, with multiple cars allowing you to make a profit. And then obviously if youve gotten some of these cars already via the wheelspin youll be able to make a nice profit.https://docs.google.com/spreadsheets/d/1l232oW9tvOxXmYNb610_...
(Not made by me, credit to u/lowjoker on reddit)