The average cost for fiberglass bay and bow windows is $2500 to $3200. The cost of installation is going to vary based on how the window is going in, is there a window there already, does it involve a roof,etc… I fair range to figure is $1000 to $3000 for installation alone.ow windows are great because they offer a bit more dimension and a unique look for any home. The average cost of a typical bow window is pretty much the same as bay windows or even slightly more $1400 to $3200.