I find that money becomes less and less of an issue if you're regularly grinding the weekly seasonals. After selling a whole bunch of cars you get from completing seasonals that happen to have buy out prices over 1 million cr in the auction house, I find myself sitting comfortably around 60 million.
I'm still spending money on parts whenever I feel like it, but the sheer number of regular wheelspins and cars you get from doing seasonals keeps your cash flow sustained.
However, this can be achieved once you amassed a handful of cars you can reliably win most of your events with, so you have a go-to car for each discipline.
But before you can do that, you'll want to build up enough cash to be able to afford the cars and parts.
The beach drag strip area is where I used to spend about 30mins every time I logged into Forza strictly to grind out cash-making on the beach. This was best completed with a specific car because of the unique combination of perks it had, but to get that car you need (probably by this time) about 300,000 cr to get the thing in auctions.