You will need about 8 cubic feet of potting soil. Garden Tower users typical spend $28 to $56 on a potting mix purchased locally.
Like all container gardens, the Garden Tower requires potting soil as a growing media (not top soil!). Plain garden soil will not work because it will quickly compact in a pot or container, constraining the root system and depriving it of the necessary oxygen that roots need to survive. Peat or coir based soilless potting mixes that contain no soil are popular, but also the most expensive. For more information check under Additional Resourcesfor links to soil sources.  What is required is that you use some form of light, loose potting soil and not plain garden dirt.  Organic mixes will cost substantially more than typical quality mixes and are not essential for a healthy system.
We recommend you ask your nursery for prices on their "professional growers mix," this type of potting soil is light and lofty and low in fertilizer content and economical. Mix in your favorite natural or organic plant food at the rate suggested on the packaging (finished compost is a great choice!) to fortify the potting mix with minerals and nutrients as you add potting soil to the Tower.
To save money, you can make your own potting spoil. Many recipes can be found online. The easiest (but not necessarily the best) is to mix loamy garden soil, peat moss, vermiculite, and compost or a good quality natural plant food.