We winterize a dozen or so boats every year. Mostly for people in surrounding areas at higher elevation. We charge a flat rate of $150. The biggest issue is draining the motor, exhaust, and cooling system to prevent freezing. However there is more to it than that. Fuel additive is used in order to prevent water from contaminating the fuel. This is very important with today's ethanol based fuels and the erratic temperatures associated with Southern California winters. It could be 30 degrees at night and 85 just a few hours later. There are several other things we do like fogging cylinders to prevent rust and contamination.
Perhaps the biggest benefit to the service is having a certified mechanic look over your boat before you put it away. If there is an issue you will be made aware of it now and not a week before your first trip when you bring it in for service.
Winterizing isn't for everybody but it is cheap insurance against a harsh winter and a little piece of mind.