Which cab do you have? the Crenlo (Factory Cab) in My A4T-1600 didn't have hinged windows, although I have seen some conversion kits.
You can Drill the glass yourself & save money. Takes a little practice, but not hard with proper bits. Same goes for cutting the glass. However, if you break the sheet, you buy another, if a glass cutter does, they replace the glass.
How bad are they quoting you? a few years ago, it was about $200 for a large Window & $75 for the corner windows when I did some checking, which isn't bad.