You need xbox live to do this. Go to the market place and for free download the free map pack from pontiac and yout get 2 campain extra levels. Go to miami alternat version and get 75,000 by killing last boss. It's easy cause you can get same money on recruit it's that easy!!!!!!!
While doing the campaign mode for the first time purchase the AUG and upgrade this most of the way, as this weapon has high damage and accuracy. Then for the sniper rifle purchase the SVD. Save your money and don't bother purchasing any other weapons as when you pass it you will unlock all the primary weapons. Then when you pass it on Professional you will unlock all the weapons. This way you can upgrade the weapons you have and also use the extra cash to unlock the Mask (purchase all Masks) achievement.