Glass cages is the cheapest for glass tanks, but for a tank that size, you're looking at about 700 lbs for a glass tank. That can strain a lot of friendships unless you hire a 5 man moving crew to handle it. If you go on E-Bay, and search for 300 gal Acrylic Aquarium, you'll find a company in CA that has the best prices, but shipping is a killer.
If you have any carpentry skills at all, I recommend building a plywood / glass tank. It's pretty easy. It's cheap, it's light and you can make it exactly the size you want. It can also be a lot better looking and very durable.
Send me a PM if you're interested, and I'll get you pointed in the right direction. I just built a tank that size and it came out beauitiful!
