This method is the fastest and most effective I've found so far to farm XP and Silver Coins and assist in obtaining the Banker and Gear Head achievements/trophies.
(amounts WILL change based on your character's level;
you will earn more the higher your level)LEVEL 100 = around 30,000XP and 300 Coins
Difficulty does not effect this method as it only changes a missions completion rewards.
Just keep repeating the steps outlined in the video. The key is by getting passed the keypad door and garage door without being detected/setting-off alarms, gives you a hefty XP and Coin bonus.