My truck also came without the block heater, but the dealer lied to me and said it had one. I didn't double check before taking delivery and like you, figured all diesels have one right. Found out when I was installing my plow mount, I don't have a block heater. I called my salesmen and he told me they would put one in for $450 (parts and labor). I ended up taking to the sales manager and he gave me a block heater at no charge, since the truck was supposed to have one to begin with.

In short, service told me it was about $250 for parts and $200 for labor. If I remember right it was something like $145 for the cord and $100 for the element itself. Call your dealer to find out for sure.