The problem you have is buying one piece of tempered. None of those glass companies temper it and you can't cut tempered glass. So they have to order it from a 'factory' and get it delivered/shipped to them. They then mark it up probably 2x so they make a profit, cover overhead etc.

It's a little high but not unusual. One of my local 'glass' shop has quoted me $100 for a normal piece of pinball glass even if I bought 10 at a time. Needless to say I didn't buy it there, but you have more options with normal sized glass.

Just be sure its tempered, with NO LOGO, ground edges and 3/16".