OC-48 can be pretty expensive. Those humongous corporate buildings and universities using OC-3 lines and higher. They can be in the hundreds of thousands of dollers. Usually upwards of a gigabit per second. I think 400 gb/s might be pushing it but they're definately fast. Most universities have a line like that and split it up among the entire campus acting as their own ISP.
The thing with buying big bandwidth like that isn't that you are buying one, you are going to buy a bunch of them. At that level any internet outage is probably going to be costing you bundles, so its safer to multihome and buy at least 2 connections. The university I go to it seems has at least 6 links to the outside world (plus Internet2).
Biggest I think any private person would buy would be a T1/fractional T3... unless of course you are crazy and run some basement datacenter.
Going to/beyond OC48 is really a "if you have to ask, you can't afford it" type thing.