While PSN has been down, what have you been up to lately on GT5? Maybe even something other than GT5? 
I have personally been cash making. Winning the American Championship Indy 5 times consecutively then seeing what is in the User Car Dealership with your new 500,000. If nothing takes your fancy, rinse and repeat and watch your cash go up. By doing it 5 times you are putting 5 x 6 new cars into the dealership which refreshes all of them since there are only 30. Also if you are thinking of doing this yourself, you should know that it takes around 20 wins to win the race 5 times (making 500,000) and that is using a ZR1 RM or Ford GT LM Race Car Spec II. So yeh it's quite good, much more efficient than doing A-Spec Endurance if you want cash (maybe even XP also).
I have personally been cash making. Winning the American Championship Indy 5 times consecutively then seeing what is in the User Car Dealership with your new 500,000. If nothing takes your fancy, rinse and repeat and watch your cash go up. By doing it 5 times you are putting 5 x 6 new cars into the dealership which refreshes all of them since there are only 30. Also if you are thinking of doing this yourself, you should know that it takes around 20 wins to win the race 5 times (making 500,000) and that is using a ZR1 RM or Ford GT LM Race Car Spec II. So yeh it's quite good, much more efficient than doing A-Spec Endurance if you want cash (maybe even XP also).