The most inexpensive way to fill a raised garden bed is to use sand, gravel or stones on the bottom 10cms of a 30cm raised bed and cover with bulk-bought topsoil and homemade compost. At the end of each season I top up the bed with 5-10cms of good quality compost to add nutrients that have been exhausted during the growing season; there is no need to empty the raised bed or to dig it.