200 sq. ft. ??? do it yourself. Do you know what \ is on it now ? Is it discolored or gray ? HD sell the Cabot line, I have heard good things about. I have never tried it because my deck 2,000 sq. ft. and was originally done with Stain from Menards, "Flood Inc" so I have been staying with that line to prevent any issues arising from non-compatibility in the stains.

You're paying mostly for labor, and the price range for that can be very wide. The only way to sort this is to ask neighbors about local prices and to get at least 3 estimates before proceeding. The other consideration is to make sure that the best, and costliest, sealer is used because it will last a good deal longer and in the end will cost you less.