Decks can be quite pricy, the materials alone cost around ten dollars per square foot the last time I checked. We build decks for anywhere from eighteen to twenty-five dollars per square foot, or around thirty dollars per square foot, including materials. If the deck isn't protected with a high-quality finish, it will take a beating. Either from the sun, or if it's in the shade mildew can destroy it.
There are a lot of choices in finish...
I tried to include the range of finishes in the flash video on the left, instead, it just shows clear wood finish (cedar tone) and Sherwin-Williams Deckscapes Solid hide water borne stain. From what I've seen, clear wood protects for about a year or two, where the solid hide deck pictured retained its beauty for up to five years with two coats. Semi-transparent stain lasts two-three years, depending on the grade you choose.