I would say that the best prices will be for twinwall (or multiwall) polycarbonate (used for conservatory 'roofs'), though you can't see through it very well. much cheaper than glass, tough and insulates better too.
If you google twinwall/polycarbonate, etc you should find suppliers who will cut it to size for you so you don't have to buy a whole sheet.
I've replaced some of my windows with it.