BIGGEST DIFFERENCE:

The cheaper "cat-back" exhausts, such as the Gibson, are not true cat-back exhaust systems. They replace the exhaust pipe AFTER the Y-collector pipe, and essentially are nothing more than a new muffler and a shiny exhaust tip.

The more expensive systems, such as the Nismo, are true cat-back systems. They bolt on at the back flange of the catalytic converters, include a new Y-pipe, muffler, and a shiny exhaust tip.

Basically, you'll get virtually no performance gain from the fake cat-backs, as the restrictive stock piping & y-pipe are still used.

From a true cat-back, you'll see minimal performance gains, as the entire piping system is being widened and smoothed.

The material differences are also there, as TJ mentioned. However, they are not the largest reason for the broad range of prices. It's the design and function that makes the difference in the prices.