Using percent of GDP as an answer to the question of "How much would that be today?" is absurd, because our economy today is not only larger but more efficient at converting raw materials and manpower into working spacecraft. The question it really answers is something like "How daunting of an investment was that, in terms of today's economy?", though even that doesn't properly convey that such an investment made today would accomplish far more.
Okay. To some extent that's fair, but on the other hand "opportunity cost to society" is just a fancy way of saying the same thing as "how daunting of an investment was that" so the quoted statement is hardly in disagreement per se :)
Of course, there are also plenty of alternative measures available at the linked site which could be better than either percent-of-GDP or CPI. (I'm leaving their use as an exercise to the reader...)
