I'm a believer in do-it-yourself solutions to simple needs such as yours. I agree that $80 is a lot of money. After contemplating my following solutions, however, you may decide that your time is worth than $80. There is something to be said for your solution to fit perfectly as opposed to a makeshift solution. Good luck with your choices.
Here's a do it yourself idea you might want to pursue. Perhaps you can modify a flexible folding sunscreen made for automobile windshields. You could use the wiring and sew a screen from netting.
You could sew a "shower cap" out of netting and stretch it over the inside frame of the window.
Make a rigid frame out of wire that fits the outside frame of the window. Glue the netting onto the wire frame. Attach with straps and Velcro to rat fur on the wall (assuming you have rat fur).