Depends on your building code to some extent. The least expensive is turf, but also the least desirable. Gravel is next cheapest, but recently became illegal in the city I live in. Next is asphalt, or some other composite material made out of recycled asphalt and tires and such. Then come pavers (brick), poured concrete, and precast concrete, though not necessarily in that order. The more they cost, the better they are in durability, load bearing and maintenance.