how much should suspension replacement cost?

I would get another opinion. Why do you want to replace the Struts/Shocks any way? There are some Silhouettes with out Air Suspension and the parts are quite reasonable. He/She asked for your VIN to look up the proper parts, which is not uncommon. The VIN has different codes. You may also be able to put on conventional Struts/Shocks and do away with the Air Suspension ( if so equipped ). This is usually much more reasonable.

Please check out the price estimator at www.repairpal.com for cost estimates for shocks and struts on your Silhouette van. The cost of rear shocks will definitely be higher when you have air shocks. With most new vehicles it is almost always necessary to have the VIN number in order to get the correct parts. With the VIN number in hand the parts person will know all of the options the vehicle has.