To reduce the cost per square foot, you can start as soon as possible. Even if you don’t have the whole budget yet, you can at least order some of them early and lock prices, as they tend to go up. Other than that, the price per square foot is proportional with the size of the house.
A small house will cost more per square foot and vice versa. You also have the option to choose less expensive materials, but they may affect the overall quality.