I recently had a frameless shower screen put in over a bath. Paid $800 inc installation, so your quote doesn't sound to bad.
Maybe try a building materials recycle place, ... they often have shower screen doors going for a few dollars. You might be able to find one the right size for the job.
I imagine most of the cost in your quote would be in the toughened glass.
had one replaced earlier this year for $386. the shower was enclosed on three sides so was only the front replaced with a new pivot screen. This was in Vic