Sounds about right to me. How did you get to 596GB? That seems very low for 300,000,000 visits (remember this was a daily figure, but you're paying by the month). If you have 10 requests per visit (reasonable for the low end) that turns into a cool 3B requests. That itself is $2,250 a month.

A million visits times 500KB = 476.84GB. Per day, so 14TB per month. At best that's another $1,600 a month.