I found out that a lot of the big window companies, like Repro or Champion, include installation in the price of the windows. They will come to your house for a "free" estimate; however, be prepared for an hour and a half song and dance about all of their most expensive windows and how you really need them. In other words...buyer beware.
Home Depot charges $99 per window for installation plus the price of the window. HD will come to your house for $25 and give you an estimate. (They don't work on commission.) If you go with them, they will credit the $25 to your bill. I don't know if that is the price at all Home Depots. I asked in Somerville, MA. This seemed very reasonable to me, and they carry Pella windows, which have a good rating.
I hope others with more experience respond too.