Might just be a bigger snowflake. What was the mass of his snowflake, and how fast was it falling? Sagan probably did the calculation, or lifted it from a pal who did the calculation, but scrubbed away the assumptions before presenting the colorfully-phrased result to a TV audience.
Post-hypertext, he could have put the calculation in a link. Oh, well.
What you really want to know is: Do radio astronomers have a piece of folklore about how much energy has been collected by all the world's radio telescopes? Here's hoping some radio astronomers will answer. (Have you thought about writing to Green Bank?)
When I was in school, Prof. Darwin Mead taught me that one erg was "the energy used by an ant when he expectorates." Never worked it out for myself, but the image always stuck with me. Barely enough to impinge on the macroscopic world.
When I came to Fermilab, I was thrilled to work out that a proton coursing through the Tevatron had more than one erg of energy. A fantastically small thing with energy enough, I thought, to make an ant stumble.