How to get 20MIL? That depends on how serious you are about getting there, and how quickly you are trying to do it.
1. Stop buying cars and parts, getting paint jobs or changing wheels. If you want 20Mil quick, the spending "gots to stop". It used to be that you could at least buy a new car with everything on it everyday, but 1.07 killed that. You can buy one every two days (although after today not with >1Mil valued cars), but doing so makes it harder to make money, since you will have to copy from the flash drive eventually and in doing so negate all the cash you made in the interim.
2. If you haven't been B-specin', you need to start. If you want any kind of life outside of this game, but still want to earn money, then B-spec is your only hope. Have Bob race while you are... eating dinner, watching tv, out with friends, asleep. Even if it's just a race every couple of hours. Bob is always up for making money. And on a side note, when you can start having more than one Bobs... do yourself a favor and start training some. They'll be worth it in the long run once you start endurance races.
3. If you are >A or B-Spec 21 go to: #4. If you aren't, then you shouldn't be worried about making 20Mil. Just follow step 1, except buy a car when you need one for a series. Try to only buy cars that can race in multiple series. And only buy enough add-ons to win the series. Trust me, you don't need a maxed out Mini once you have a couple of Prancing Ponies in the garage. Race every series, 1 after the other until you are a 21.
4. If you are an A or B-Spec 21, the most efficient way of making money I've found is the "Like the Wind" Indy race. If you've followed my step 1 then you have enough money to break it and buy a GT Formula car. Every 3:30 you can make $70k in A-Spec, a little longer in B since it is more laps. Yes, that's right, add it up. You will need to run that race about 286 times to make 20Mil. Thankfully, in that time you will eventually grind up to a 23.
5. If you are now an A or B-Spec 23 there is one race that can give you a slightly higher money to time ratio, that being the "American Championship" Indy race. Can't use the Formula, so now you get to break rule 1 again. Any highly modded Ford GT can easily win this, I've also had good results with a RM Corvette. Now you are making around 98K every 4:30 on A-Spec, longer for a B since it is more laps. Assuming you've maintained at least 10Mil getting to a 23, then a short 103 more races will get you the rest of the way.
And this my friends is why we dupe cars. Because after going through all that, I personally feel like I should be able to buy all of the Ferrari Formulas, the 330, Muria, XJ-13, MK IV, and well just about every other multi-million $ car in the game. Certainly I've earned the right to buy them without going through all of that 4 more times. Some may call it cheating, I call it time management.
I didn't even buy a PS3 until Jan 13th, 2011. The only reason I bought one was for this game. I rented my first copy of Gran Turismo right after it came out and never took it back. Played every one since. Took a week off work when GT4 came out, didn't have the luxury with GT5. I'm currently a 28 A, 26 B. I work 40 hour weeks, and when I'm at home and the wife is not (or is a sleep) I play GT5.
And why might you ask did I take the time to write this rather than play? Because I'm waiting for my last multi-mil trade to come through before this ridiculous ban goes back into effect. Thank the lord this is the last of the $10Mil+ cars I needed to acquire!
OASN, I realize there are a few ways around some of the little details in my "plan". This was meant more as a basic course of action for those serious about making 20Mil on the quick. Besides, time will teach you those, and if nothing else... GT5 will take time.
So to those still reading, I say good luck.
I use the American Championship for some serious grinding, as the event gives a payout of 97,000 Credits. Unfortunately, the race gets very tedious after prolonged driving. So, I'm trying to find 
