This is ridiculous! I spent the whole day trying to get 500,000 credits and I am NOWHERE NEAR. Any chance someone can send me 100,000? One shouldn't have to play day and night to complete weekend tasks.
The easiest way is by doing Fire stations in the beginning as those missions pay the most. I recommend around 20 fire stations before venturing out into police and medical. But it is slow going at first. Stick with it as it does get easier.