This is a great paper that makes interesting reading as a probe of the effect of various demographic factors on attitudes about the future and technological advance in general. But the paper doesn't show that most people overestimate cost. The distribution of cost estimates is normal in log space, with the mean log being pretty close to the right answer.
Some people overestimate while others underestimate, but the median estimate is about right.
I don't think Scott looked at that. The raw data is included with the paper, in case someone else wants to look into such questions. It's probably easiest for me to respond to this. I did some supplemental calculations on new year's eve after returning from a party. The overall correlation between the two sets of responses are therefore statistically non-significant.