How much to replace the intake manifold?

What is your question?

We've had a coolant leak that we haven't been able to track down, and the check engine light has come on. Our mechanic told us that the code is for the intake manifold & it needs replacement. We just put a transmission into this vehicle and don't need another $700 of repair costs! How can we tell whether it's just the intake manifold gaskets that need replacing, versus the entire intake manifold? How much should replacing the manifold itself cost (along with the gaskets, I'd assume)?