The square foot gardening book recommends 1/3 each peat, compost (from various sources), & vermiculite. The mix is well draining & does retain some moisture as well.

Easiest would be to use bagged potting soil & mulch your planting with compost.

Cheapest would be to use garden waste, fall leaves, etc. to layer lasagna style then plant your annuals in spring. It will reduce volume once composted, so you might need to top it up in spring with potting soil or more compost.

Thanks!