if u want to have your own customized aquarium, u will have to prepare to fork out money mate.
doesnt matter u order direct from LFS or glass shop, its goona be costly for non standard tank.

If u wana know why let me explain to you in a simple example, the figure are juz a rough estimation for your understanding only.

let say one BIG piece of glass can make three (3) 3 Feet Tanks.
now because of your customized tank this piece of glass can only make one (1) 3ft tank plus your customize tank.
so who is going to bear the off cut ? obviously you 

in short, you prolly have to pay the price for 2x3ft tanks due to the material used/wasted.
and then the manpower, for normal standard tanks they already have the frame where they juz need to slide the glass in and silicone it but for ur customize tank prolly they dont have the measurement so extra effort is needed to complete it.

which is why its much more costly compare to normal standard one, and of coz the LFS would want to earn some profit as well so prolly that will add another 10-20% of the cost.