Around $35 for the block heater. My dealer charged me $65 for the heater and installation on my new machine. I figure if those guys can read the installation instructions, so can any one of us.
The block heater will cut down on the amount of warm-up time once the engine fires up. If you've ever started a cold diesel engine on a sub-zero morning, you see a lot of white smoke. In those cases, I don't go anywhere or do anything until the white smoke clears up, and sometimes it can be 10 minutes of idleing before that happens. Again, as most said, a block heater is probably more of a piece-of-mind thing.