[atlas] Credits System

Hello, I was wondering if you please explain how the credit system works, specifically traceroute measurements. I performed 2 measurements yesterday using large packets(1500) and targeted 500 probes per experiment. When the bill came it was rather shocking. I realize this is a tall order, but is the amount correct? The measurements I'm referring to are : 1043292 and 1043293. They each totaled to 1M credits, should it not be half of that? Kind regards, Warwick Louw