3/8" four feet by five+ feet at $150 doesn't sound horrible, actually.
I understand that building your own tank is a worthwhile goal, and I understand and support that kind of DIY project. It's amazing to look at something and be able to claim complete ownership. Just keep in mind that a tank is a small (and sometimes tiny) fraction of the cost of a complete setup. Stand, lighting, substrate, filtration, CO2, flora and fauna massively increase the total cost.
That being said, good luck! As long as you're doing it for "love of the game" rather than cost reduction, you have my 110% support.