Monthly lease payments are calculated based on how much a car depreciates over the course of a contract. Two factors which impact each deal's price massively are your annual mileage (how long you decide to drive your new car each year) and the length of your contract (how long you decide to have the car for).
In short, the more miles you specify on your deal, the higher your monthly payments will be. This is because a higher number of miles driven means there's a greater chance of there being more wear and tear on the vehicle.
However, often confusingly, a longer contract (e.g. 48 months) will show a cheaper price per month than a shorter one for the same car. The reason for this is because finance providers recognise that the most a vehicle depreciates is in the first two years of driving. A three or four-year agreement, on the other hand, spreads the cost of depreciation is therefore less each month.