This is a bit misleading. They throttle you to 256kbps after 15GB, but give you the option to lift the limit by paying $10/GB for the data you use past 15GB in a given cycle.

It's literally impossible to reach 250GB without paying extra. After being throttled the most you'd be able to download is 82.94GB, assuming you were downloading things non-stop at the maximum speed for 30 days.