Maybe you could be of some help to "with my thinking" via you experience.
You are correct that at about 50 to 100 gallons the $1/gal rule of thumb starts breaking down starts going to the 2nd power of the tank size but:
(and bear in mind that I have never seen one of these).
As a 30" deep tank is "real pain" although IMHO necessary for full growout of angels (and possibly also discus) and therefore the remainder of this post is based on a 30" deep tank.
Also for purposes of this post it is assumed that the tank will be placed against a wall and not freestanding in a room.
The tank would be 12'x3'x30" => 650G.
Obviously a glass top which is 12'x3' will not work if for no other reason just due to it's weight so:
let's break down the tank into 4x3' lengths.
the angles which the top pieces fit into transversely will also function as structurally for the longitudinal walls (this is where the big glass cost savings "kicks in").
Each top panel would be doubled hinged:
one hing at 8" from the front of the tank and
the other hinge at the middle of the panel.
Each panel would have two of these on top: http://www.marinedepot.com/ps_ViewIt...t~CU01014.html
The above is just a very sketchy outline
but sets forth the major elements, IMHO, of the structural portion of the tank construction which results in major cost savings.
I have "partially mapped out" sketchy outlines for the construction of the canopies, overflow weirs, return nozzles, sumps' design, etc if anyone is interested.
BTW: this design is very scalable: you want to have an 850G tank just add 3' to the length of the one described above.