There are a number of heat shields on the exhaust, and their prices vary
from about $20 to $50 each. They rust off at the attaching bolts.

There is also one in front of and one behind the exhaust manifold.
Replacing the rear one requires unbolting the manifold from the head, and
the danger there is that one of the studs will break off, making for a
major headache.

It's easy to determine which one is making noise. All you have to do for
the ones underneath is to raise the car, get underneath and wiggle/tap
them. The rattler will be very apparent.

If repair is required, this can be achieved several ways:
1) Pull them off. Cheapest, quickest way, but not the best.
2) Replace. Best, but most expensive way.
3) Clamp in place so they don't rattle. You can usually successfully use
large hose clamps to this end.

As to how much it will cost? You'll have to find out exactly what's wrong
first. It might not even be the heat shields.