Play sand, pool filter sand or black diamond blasting sand will be the cheapest you can buy. Play sand is very fine and natural looking. Pool filter sand is very white looking and imo less natural looking, but is easier to clean then play sand. BDBS is obviously black, and again much chunkier then play sand.
All are very good, and common substrates. I would look around your area and see what you like best, as sand does differ from area to area.