I'm sitting on 7mil at the moment. I rely mostly on daily workouts to get my cars, and I have no intention of grinding for the 20mil Ferrari and Jag.
Currently I have 131 unique cars, out of a total of 193 (according to the Master Car List here). I only bought 31 of them, rest are prize cars or Daily Workout cars

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Most expensive car I bought was the Cobra for 2mil, and since then I have got 3 more of it through the Daily Workout

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So I put a lot of faith in the GT Gods lol
EDIT: Just rechecked, and my memory was playing me. I DID win the Cobra in the daily workouts, plus 3 more later haha. So I haven't spend over a mil on a single car then, most expensive being some Gr.3 cars. Let the game do the work