How can I find out how much a passenger window of a Boeing 737 costs?
This page has declared values for part shipments, including one with the 140N2139-4, which is the assembly containing the seal, mid pane, and outer pane for the 737 NG. The declared value is 24,562 INR which is currently about 375 USD. A shipment of 20 also listed has a lower unit price.

For searching purposes GKN has part numbers in their catalog. You typically have to ask for a quote from suppliers.