Looking at the requirements to this challenge, the minimum required would be 5 each of common, uncommon, and rare maxed Renaults. I only have couple of these, so I went back to my cost of upgrading post (https://forums.hutch.io/discussion/comment/20431) to figure out how much needs to be invested.
The cost of maxing cars specifically for this challenge is $346,375, assuming (1) you don't need any different cars for stages on the same rarity and (2) you already have the cars you would need to do the fuses. If you don't have the cars laying around and have to buy packs for them, that goes up to $566,750.
The total rewards for the challenge are $58,500, and I'd go ahead and say that most of these cars will not be very useful to have upgraded going forward.
So Hutch, I've gotta ask - did anyone do this math? If so, what was the logic? If you have to max even just one of each class to complete the challenge, it no longer pays for itself. I sure hope that nobody did the math, because otherwise it looks like you're intentionally conning less analytic players out of their resources...
Edited for a better title and clarity