Having worked in the glass trade 30+ years ago for a glass supplier. Back then there were a few suppliers. Guessing there are fewer today since even the one I worked for shuttered their doors years ago and we covered all of LA and Orange county back then. They all get there glass from the same maker/s. So the difference comes from their mark up and how much they want to make or need to make to meet bills.
I can tell you that some, like probably in any trade, give a high estimate to make you go away thinking you’re not serious or just don‘t want to deal with this.
You’re prefix is San Diego. Find a Mexican shop and get a quote. They may be more reasonable and easier to work. It for the money, time, and aggravation find something close to the size you want and settle. By the way, what dimensions are you wanting?