The commission shall establish and collect rentals, which, with the proceeds of sales, will yield an amount sufficient:

1. (a) To pay the costs of administration and operation of the authority;
2. (b) To pay the costs of maintaining, operating and improving its properties;
3. (c) To pay the interest on its contracts or bonds;
4. (d) To provide a fund for the principal of its contracts and bonds as they come due.