"You have dismissed Mars mission cost estimates I have provided over and over as unrealistic. It's easy to dismiss me as a naive outsider, but what do you have to say to Oberg when he makes similar comments?"

Oberg did not give a cost estimate. The point of the article was to show that other pundits weren't giving accurate assessments:
"Most space historians would say it’s Easterbrook who is peddling nonsense. When he started with a 1989 figure of $400 billion for the elder President Bush's Space Exploration Initiative, Easterbrook wrongly assumed it was only for a Mars mission, and then rounded up for inflation to arrive at a figure of $600 billion. A lunar base would cost another $400 billion, equaling the magic $1 trillion. But he counted the moon base twice, it turns out.

According to space historian Dennis Powell, this figure "apparently derives from the half-trillion dollars NASA (kicking up its heels in delight) estimated would be the cost of a gold-plated moon base and trip to Mars when George H.W. Bush proposed it in 1989. Someone, somewhere, applied some goofy mathematical spinning to the price of NASA's 1989 wish list and came up with the $1 trillion figure. It's worth remembering that NASA didn't get its wish list." "

"Bush’s new space plan certainly deserves to be debated. And it won’t be cheap. But any discussions that are based on flawed data and outright fantasies are worthless."

My guess is that Oberg would be more on "Larry's side" rather than yours in this supposive "debate". But as far as the article itself, I have confidence that Oberg is providing a fairly good account of historical space program assessments and I doubt Larry would argue with it.
[and of course I agree that Easterbrook is clueless on space issues].