We don't keep track of materials prices or know which suppliers provide the cheapest materials, but as for materials to use to fill raised beds, using homemade (or bulk-sourced, such as from local landfill facilities) compost can comprise a significant portion of the filling. Topsoil could be another additive, if it's of good quality (refer to the bottom of the linked page for more information.) Ideally, depending on the depth of the raised bed, these additives can be blended into the soil below, which will not only improve its drainage, but give you more soil to work with, as it will be looser and take up more space with the amendments mixed-in.

Whatever mix of ingredients you decide to use, we do recommend avoiding putting large, bulky materials like logs, branches, sticks, or rocks at the bottom of the raised bed in order to fill space. This type of layering creates drainage and nutrient availability problems that may put too much stress on the plant roots.

Miri