What's with the whole window thing? Why am I bothering with windows?
Let's approach from a different direction: increase of value to your house. 

Any realtor or homeowner will tell you that certain things add value to your home, and others do not.  Even for things that add value, you will only expect to recoup "most" of the cost (since you are doing it for yourself and will only sell to someone else eventually).  For evidence, google the phrase "recouping cost of home improvement", and note that a well-done kitchen remodel of 20k will add maybe 16k to the value of your home.  A well-done addition or remodel is one which 'improves mass appeal' of your house, because then it's easier to sell and more people bid on it.

With all that in mind, I say:
skipping the window does offer short-term savings of a few grand or so,
but adding the window is an investment that makes your whole house worth more.