I got a quote from them last year for a 12x5x36" tall. I almost fell over. Just and with overflows no shipping $22,000 I know once you go over 30" tall the acrylic needs to be thicker but damn that's a lot. It's cheaper for me to add another 750g and set them side by side
Taller than 30" definitely gets expensive because any reputable builder will use 1"+ thick acrylic, but the length of the sheets will also be a factor. An 8' sheet is standard so it's not too expensive; you pay a premium for jumping up to 10', but when you go over 10', the price jump is pretty huge because that's an even less common speciality size. I bet if you went with a huge square (8'x8'x3') it would cost a lot less than $22K and you'd end up with more gallons...but I like your 12x5x3 dimensions over 8x8x3 anyday.
ATM does make nice tanks. I was looking at a used ATM tank (a 14'x3'x5' made from 2" thick acrylic) a couple years ago and the seams were flawless. The shop selling it said it was Dr. Dre's old tank and it must have been 10+ years old . They were only asking like $5K for it, but I couldn't figure out how to transport it without costing me a fortune (the tank alone weighed over 3200lbs!).
Their tanks are expensive, but you get what you pay for.
I wish I had taken pix of the tank, but here's a pic I found on the internets...it was an in wall tank with huge overflows on each end.