depends on the quality of windows you are looking for. the cheap ones at home depot will run you about 100 per window. But then you might want to check out Pella and see about the triple paned windows or ones that have gas inside to help keep the heat in your house.
The thing with the window replacement companies is that they do the installation quickly (probably a couple of days) a contractor, based on number of employees might take longer to do.
You might save some money by buying the windows, but contractor vs. window guy will be probably the same. to truly save, you need to install yourself.
Maybe go back to the window guy and say you got a contractor to do it for xxxx dollars and see if he matches it….. As long as they are making some kind of commision the will make the deal.
I would consider the low end to be about $500.00 per window. Very expensive.