Isn't the simplest form a tree?
The best "premium shadow package" available on the market. Comprises: filtered light, relaxing breeze sounds, noise blocker, chirping birds and flowers, dumb squirrels frolicking around and fresh soil, all in the same package for just ten dollars a sapling.
In any case, hedges don't need 20 years to grow.
Will depend a lot on species, age and season of the year. Conifers are more expensive because can't be sold at fall as naked-root trees.