Since you said exhaust manifold, I assume you are talking about a Zetec. It really would help if somewhere, profile, signature, or in the post if you sould tell use what every parts counterman must always know to get you the right part. MAKE, MODEL, AND YEAR as well as other DETAILS that will help us know what you are asking about.
With the assumption (assumptions are dangerous) that it is a 4 cylinder, the exhaust manifold includes your cat. It isn't particularly cheap.
An exhaust leak anywhere near the oxygen sensor can throw off the oxygen sensor readings and create engine drivability problems and also may lead to setting check engine codes. An exhaust leak is also dangerous for the passengers of a closed car in that it can lead to high levels of carbon monoxide.
Sometimes such cracks can be welded. Sometimes you need to replace the manifold.
So, please tell us MAKE, MODEL (INCLUDING ENGINE SIZE), AND YEAR so we can better help you instead of just guessing.
