My calculation was simple. 325,000,000 (approx population of the US) * $20 == $6.5 Billion. Trump shut down the government over a $5 Billion budget request (plus or minus, he hasn’t always been consistent with the figure), which is close enough for government work.
Of course $5 Billion won’t cover the cost of the wall as proposed, but it would theoretically get the government back open again.
And of course I’m expecting everybody to get the absurdity of asking kindergartners to fork over $20 to unblock internet porn.