The least expensive exterior siding is a plywood product called T-111. It comes in 4'x8' sheets, installs over standard building paper, and is by far the least cost. It comes in 3 styles, vertical groves on 4" centers, vertical grooves on 8" centers, and no grooves. Each sheet retails around $32 depending on the time of purchase. That gives you a material cost of about $1.30 per square foot after adding for paper, staples, and trim.