Stage 8 and on requires Toyota Lemans car and it costs 800 GC when it's on sale or 1000 GC regular price.  That's $80-$100 real money.  I have not spent a penny on this game and I don't plan to spend any in the future either.   

I had saved over $5,000,000 by repeatedly racing in Accolade Open's last race for the first race of each day.  With the 2X R$, the winning prize is over $60,000 in 8'25".  If I have a free double agent, the prize doubles to $120,000+.