Try updating your code like so,
var moneyDropper:int = 0;
private function timerMoney(event):void {
if(moneyDropper == 0)
{
drop500.start();
}
else
{
moneyDropper += 500;
}
}
private function dropMoney(event):void {
money.text = String( Number(money.text ) - moneyDropper);
}