For the instalment plan, we don't need to get financing elsewhere since it's the clients who are financing the project... But for the mortgage plan, the client will get a complete house only by paying 20% deposit, meaning that we'll have to finance from other sources and hence have some cost of money implications... That's why we have a 1 m difference....
Note, the 5.95 m price is a discounted price hence the comparison should be the 6.95 m price