I'll be interested to see how that cementboard holds up, if you go through with the project. I built a 200gal plywood tank over the summer. The only thing I would be concerned about is its brittleness...I'm not sure how much water depth the stuff would handle, but it seems pretty sturdy. The glass I got locally was 1/4", a pice about 60"x18" and cost me a little over $50 so your estimate is probably pretty close.
What kind of construction method are you going to use - seal teh boards individualy then seal them together as you would a glass tank, or is there a method of joining them together to form the box and then sealing the inside?
I was planning on putting it all together sealing with the Quad, then Drylocking the finished product.