If your X is registered in California, you can take advantage of a State Law that all emission related parts on the car have an extended warranty of 80,000 mi. You do not have to have purchased extended warranty, it's an auotmatic warranty mandated by the State, and the manifold falls under this rule, along with the cat. converter, etc.
Just take the car to the dealer and SPECIFY TO THE DEALER THAT YOUR CAR HAS A CRACKED MANIFOLD THAT NEEDS TO BE REPAIRED.
I got mine replaced for free!