It could very well be that they only charge by intervals of 30$/GB; meaning the minimum that they are willing to charge is 30$ but he could have used up to one

It could very well be that they only charge by intervals of 30$/GB; meaning the minimum that they are willing to charge is 30$ but he could have used up to one gig. It's common enough for U.S. Carriers to do this. No comment on whether that's a fair price.