What are you actually going to be using 20TB on?? If your using that much data you should really be looking at dedicated servers
High resolution product images.
JPEG 2000 would help. So would HD Photo, but JPEG 2000 is not that widespread yet and HD Photo is not even a standard, much less supported. The target is a flash object. Now that Adobe has bought Macromedia, I'm looking forward to JPEG 2000 support in flash,
but that's besides the point.
During peak loads it is entirely possible to suck down a lot of gigs of that monthly quota. Either way we can't really pass on the transfer bill to the customer.