Make Money
You can repeat this as many times as you want:

1: Load career profile.
2: At cheats menu, enter UNLOCKALLTHINGS.
3: Save your career.
4: Reload career, go to garage.
5: Sell unlocked bonus cars for cash.
6: Repeat.

Enter these at the cheat/code option:

1MA9X99 - $2,000
1MI9K7E1 - $10,000
CASHMONEY - $10,000
CASTROLSYNTEC - Castrol Syntec Vinyl
ENERGIZERLITHIUM - More vinyls
HORSEPOWER - Chevelle SS
ITSABOUTYOU - Audi TT
L1IS97A1 - $8,000
LEIPZIG - Progressive Bonus Vinyl
MITSUBISHIGOFAR - Mitsubishi Lancer Evolution
REGGAME - $10,000
SAFETYNET - Five Repair Tokens
UNLOCKALLTHINGS - Nissan 240 SX, Pontiac GTO, Chevy Cobalt SS, and Dodge Viper SRT 10
W2IOLL01 - $4,000
WORLDSLONGESTLASTING - Dodge Viper SRT10
ZEROZEROZERO - Coke Zero Golf GTI