Acrylic should keep the cost down, but can be difficult to work with, especially gluing it together. You get basically one shot to get it straight because the chemicals used to glue acrylic together essentially melt it together almost instantly.
The reason I asked about building a tank and then framing it are because sealing acrylic to wood is extremely difficult. Silicone doesn't really like to adhere to acrylic. It tends to pull away and weaken over time. So an acrylic tank framed in wood would be the best way to go.