The cheapest way to renovate a wooden dock/pier/deck is to merely flip the boards over and screw them back down. Yes, you may have to replace a few boards due to cracking and warping, but of the several decks/piers I have worked on and repaired, the boards look almost new on the underside. Two caveats: they have to have been screwed down in the first place, and the wood has to be pressure/weather treated.
