Profitability of farming wheelspins: 100 Super Wheelspins results
Cost: 8.6M in Subaru 22B's (100 cars), 3000 Skill points
Time: 3 hours total. 2 hours spent farming points in events. 1 hour for buying all the cars, upgrading all the skill trees and removing them from the garage.
Profit: ~3-4M (~12M gains total), a bunch of rare cars, and a few Forza Editions that I will gift.
Best cars: Ferrari FXX-K Evo, Mclaren Speedtail, Rimac nevera, Jaguar Lightweight E-Type, Porsche Prodrive rally raid, Pagain Zonda R
Disclaimer: I bought every purchasable horn/wearable beforehand.