Glass cleaners special for cars are always way to overpriced. I once bought Autoglym Fast Glass, did the job perfect for €9,50/500ml. Now i use Frosch bio spiritus glass cleaner and i also get perfect clean, streakfree windows for €1,75/500ml. When the windows are not very dirty, just 1/3 white vinegar in 2/3 water does the job as well.
Before the Turtle thing (earlier post) the windows are twice a year cleaned with Poorboys professional polish, which is a cleaner polish (without abrasives) for paint, metal, plastic and glass.