The cost to fix an exhaust manifold leak depends on where the exhaust manifold is leaking.
If the exhaust manifold is cracked and leaking then it either needs replaced or welded and repaired.
It's usually cheaper and easier to just put a new exhaust manifold on the vehicle than it is to remove it and weld it back up etc.
So that charge could be $200.00 or more for a shop to replace the exhaust manifold with a new one.
If you replace it yourself then it just cost you the price of the exhaust manifold which you can buy pretty cheap online and usually for $200.00 or less.
If it's an exhaust manifold gasket that needs replaced then you can replace the exhaust manifold gaskets for usually less than $50.00 for the parts.
It's not that hard usually to replace the exhaust manifold gaskets as you just unbolt the exhaust manifold and remove the old gaskets and put new ones in there place and bolt the exhaust manifold back together to the engine.
But if you need a shop to replace the exhaust manifold gaskets it could cost $150.00 to $300.00 for the labor charge and the mark up of the parts.