get in a fast car and drive like the wind! only smokin' with ya!
try these two options (both take about as long as each other to complete).
1. get any car and race the second easy special condition track (costa di amalfi). do it forward and reverse, and you win the toyota rsc rally raid car. apart from the 10,000cr, this car is worth ~250,000cr. race till your bored or you got the millions you need. there is a bug in the rally road sections of gran turismo 4 - there is no tyre wear. so slide into a set super-soft qualify tyres and own the track!
or;
2. get a dtm car. you can do option 1 (above) twice, and then buy an audi touring car either will do. then b-spec the dtm chapionship at x3 speed. its 15,000 a race, 75,000 for the championship, and if you win, you are awarded the clk gtr which can be sold for ~763,000cr. if you win every race in this series, you can get almost 900,000cr. the only stumbling block to this is that you will have to train up b-spec bob, but once he is up to speed, bobs your uncle! (so to speak).