Unfortunately, this is one of the worse problems to encounter on your particular engine. (3.5-S.F.I.)
The crossover manifold actually contains 3-gaskets that cost a little over $8.00. But you must tear down half of the motor to get to them. (There are NO short-cuts)
The following parts must be removed: 1) Intake manifold/plenum
2) Exhaust down pipe
3) Exhaust manifold
4) E.G.R. Valve
* A few more small/related parts*
All of these parts require new gaskets once they are removed. Trying to replace these parts without new gaskets will guarantee a leak in that system, and possibly a blown motor.
Alldata, a repair guide, quotes this as a 6-hour job. Which means a minimum of $600 in labor alone. The gaskets will run you another $100.