Have you posted about this before?
Do you have a ridegevent? Soffits? I know we talked about soffit vents. They're little grates under your eaves/soffit that allow airflow up through your attic. There should be a ridge vent at the peak of your house if so.
There really isnt a way to give you an estimate. We have no idea what you've got going on up in your attic. Call around, look in the classifieds/online for carpentry prices around you. Most carpenters work in all conditions. Winter is when mold breeds ( i believe, warm and cold air meeting) do you have any black mold in your attic? How about directly above where your bathroom is? (In the attic)
A plastic gable vent costs nothing, but setting ladders up and doing all the labor will cost you. Depending on the contractor, could be a full day of work. Also dependant on the siding of your house as well. Vinyl will be cut, and j-channels will need to be added.