I would suggest saving the money and do it yourself. I used to work at a few marina's at the coast and one of my jobs was to winterize boats that we stored in the dry stacks. It would take about 15-20 minutes to winterize each motor, if i remember correctly. However it has been so long since i did one i had to google it to remember all the steps. Here's a link that tells you what to do, better than i could. It says it will take 2 hours but they include cleaning and waxing the motor and cowling etc. Pretty much all you will need to buy is some fuel conditioner, fogging oil, new plugs(for next year), lower unit oil, and a pump to replace the oil with, any oil or fuel filters you may have, and grease. This is nothing compared to the gas and time spent hauling it to a mechanic who is going to charge you an arm and leg for 20 minutes worth of work. Here's the link...
HowStuffWorks "How to Winterize an Outboard Motor"