You earn money for completing Hitman Contracts in Contracts mode, these can either be from ones you search for, are given from by IO Interactive as you play missions or from friends in "tournaments".
They're all for differing amounts of money depending on what is required, but it's very quick to earn $1m credits, for example, simply choosing:
Mansion Ground Floor (the first level) hide behind the basket to your left as the guard comes down the stairs, sneak past, go upstairs, garrote the guard at the balcony (you may have to wait until he starts giving his talk) and put the guard in the basket in the bathroom (at the top of the stairs) then run to the exit (on the balcony).
This is a one hit mission, simple garrote kill, bodies hidden, unseen. Set this for a friend, it takes less than a minute to play and will earn them around $350,000, which is considerably more than other missions with multiple, difficult hits will net.
Simply adding more things to the mission (more hits) will obviously add to the funds.
Note: those who make the Contracts do NOT earn this cash from it, have your friend make the contract back and invite you into the 'tournament'.