Best way to get an accurate pricing idea is to check with one of the (many) on-line aquarium/custom aquarium shops. They also usually have standard sizes. I had a custom/cut down 50 gallon made in acrylic for an ongoing research project. The tank $$ was reasonable, but the shipping was killer! Best if you can find a regional shop that you can drive to if you get serious about purchasing. Shipping by truck freight can add a chunk to the cost.
I used Glasscages.com
and they were very accommodating for my custom order. An acrylic tank would cut the weight by a huge amount compared to glass, but you'll have much more reinforcing across the top and around the top inside. A quick check on their site: a 270 gallon acrylic runs a cool $2898, a glass 300 is a mere $1470, not including shipping.
With a custom job, in glass or acrylic, you can shape it to fit your space. You are probably looking at a custom-made stand anyway!
It's fun to dream, if you're going to dream, dream big! That 50% weekly water change would not be fun though