Regarding legal costs, Lessig has made up the $1MM figure apparently based on nothing, or (at best) his intuition. He also didn't say the defense would cost $1MM. He said Aaron could have feared it would cost that much.
As for sentencing, in cases like this, the sentences for the different charges would almost always run concurrently. We're talking a few years of imprisonment upon actual conviction, and reportedly the government offered to settle. Aaron could very likely have gotten a suspended sentence ("probation") upon settling. The judge in the case is known for harsh sentencing and favoring the government, but even then, it's hard to predict what he'd have gotten even if he had been convicted. My best guess, as a techie lawyer, is 12-18 months. The rhetoric here is all just Lessig's attempt to get people like us riled up, and I'm tired of that.
Not that I'm defending the government either. I don't like the law here. I'm asking only for some perspective. A rich or at least middle-class white kid knowingly and purposely engaged in an act of civil disobedience and got caught. If he'd looked different, the state trespassing charge probably wouldn't even have been dropped.