Prices can vary widely from supplier to supplier. Try to find out if there are any insulated glass manufacturers in your area and ask if they would consider a direct sale to you. Remember that a 4 by 8 foot sheet of glass is very difficult to move without a proper glass rack and some experience. You may want to inquire about cutting it in half.
I have a question what size viv are you making? You a 100 bucks in, so just wondering how cost effective this project is going to be.
I seems like your building a 75 gallon tank.
I am all for doing it myself , just wondering.