jman - I think Bill is right to some extent. Unless you are trying to do something like low iron glass, you are going to have a hard time beating the economies that you see in off-the-shelf tanks. But, by using serious reinforcement around the top they can also make their tanks thinner, further reducing cost. I'll bet if you take calipers to a 90 in a store, it will be 3/8", not 1/2". And 1/2" cost disproportionately more because it is far, far less commonly used than 1/2".
But I do suspect your prices are high. I'm getting my Optiwhite at a real bargain rate at around $15 a square foot. I think regular 1/2" float glass like you ordered should be able to be found for maybe $10 to $12 a square foot, cut and edges polished. So do the math. But do keep in mind that they order in standard widths, 60" being common, and if they have any wastage due to the sizes you order, you'll be paying for that footage too.
Learned more about glass than I ever wanted to know on this little project...
Steve - 33g reef and a 180g planted in need of a re-scape.