It can range between $250 to $450 if you have dual engines. That tends to cause most owners to do winterization on their own if possible.
Check a search here or a shop manual for procedures. Not that hard really and generally includes blowing out exhaust and injecting anitfreze, pulling plugs and fogging motor, adding fuel stabilizer to tank, tending battery for season, some minor lube spots, and decent cover/storage. Many swap out plugs, do oil and filter, and will run through BUDS depending upon usage and hours, some do these for pre-season launch. Good idea to flush out bilge and spray down with WD40 to limit moisture in storage. There's other items but those are most of the should dos, some vary with craft.