That honestly sounds about right price wise for 3/4". Your glass thickness is calculated mostly by the longest deepest point (36"×36"). If you want to decrease thickness you'd want to make the tank hight shorter as it's the dimension that changes thickness the most . Ask the glazier what safety factor they are using for the calculation as well. Most professional aquarium manufacturers use a safety factor of .4 btw. With a correctly braced tank built by an experienced person that would be 1/4" thick glass for the dimensions you listed however if you've never built a tank I'd absolutely increase the thickness.
The glazier you deal with will also likely charge you extra for the complicated cuts involved with building a drop off and finishing all the edges. The edges that meet where the drop happens will need to be carefully planned and detailed to be cut correctly and notched in the right places. Most tanks have three surfaces interconnecting at the corners a drop-off has an added intersection where the drop takes place that needs high accuracy and thought as how you're going to make the connection. I built my current tank (drop-off) in acrylic, it's about 150gals the acrylic itself was about $2000. A large cost for these materials is the shipping so if you can pick up your supplies it saves a lot. There are also sometimes standard sizes that a shop will work in so you can always ask if there's a certain dimension that's increasing your cost. Building a small custom.tank like this will absolutely cost more than buying a similar size one btw. Build thread is in my profile if your curious.
