Where do I have to look to 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.
Assuming that you are referring to a Boeing 737, here what you might be looking for:
But as these are made-to-order (as I mentioned in my comment), there is no MRP / MSRP - you have to directly contact them to get a quote. The actual unit price depends on your application, the quantity of windows you need, and other such factors.
EDIT: If certification isn't necessary, you might as well look for aircraft bone-yards, where you might get windows, which will be way cheaper than certified, made-to-order units. (Courtesy: @DanPichelman)