Window installation cost only.
I don't want to tread into this mess, except to say that our contractor was watching the large McMansion being built next door and noted that all the windows were installed in one day. Perhaps the home owner thought that was great and speedy, but our contractor couldn't believe all the things they didn't do -- like checking to see if the windows were level and plumb.
(Their gutter guys were equally as speed -- the gutters leaked all winter and caused ice damning!)
Anyway, quality installation is important -- so always consider that with any attempts to be cost conscious!