We did a window shower a month ago and used the cheap $70 surround. Next year we are tearing into it and removing the window and replacing the whole works, so we didn't want to spend much this go-around.
I HATE windows in showers. They have so many problems that it is well worth it to remove them. Several years ago our solution was to install glass block windows, but now we frame the hole, side the outside, insulate and sheetrock the inside. No more window.
My in-laws have a surround that is made up of large sheets of some type of engineered marble-looking product. Each wall is one sheet. I wonder if that would work for you? I have no idea what it costs though.