I think $10/GB would be reasonable considering that they charge $30 for 3GB.
I think $10/GB is ridiculous; in South Korea, you can buy 1Gbit/s for $20/month - which would take you about 10 seconds to hit $10.
Given that there are 60 seconds in a minute, and 60 minutes in an hour, that's about $360/hour, or $8,460/day, or to put it another way, a quarter of a million dollars for February, and more than that for other months with more days in them.
Tell me again why they are selling other people's packets as if they were metering water, as opposed to renting us pipes for those packets based on pipe diameter, and getting the hell out of the way otherwise?