Last year when I had my thermocouple replaced on my natural gas furnace it cost me $150.00 including the service charge.
The thermocouple can be replaced cheaper if you know what you're doing by replacing it yourself.
But I decided to just hire it out last winter because I didn't want to mess with it and needed the furnace fixed fast since it was in the dead of winter.
My furnace pilot light kept going out and it was the thermocouple causing the pilot light to go out on the furnace.
You can buy a new thermocouple for $8.00 to $15.00 at your hardware store and a handyman can easily replace it as well if you can find a handyman.