Studies show that the U.K. leaving could save roughly 8 billion pounds per year, according to Cramer. Meanwhile, Norway pays roughly the same amount per capital for access to the European market, and Norway isn't even a member of the EU. So, the idea that the U.K. is saving any money seemed fanciful to Cramer.
"But let's be honest, Brexit was never really about the money, it is about the immigration issue," Cramer said.
The U.K. has let in approximately 200,000 immigrants per year in the past five years, Cramer said. As of 2014 there were 8.2 million foreign people living in the U.K, which amounted to about 12 percent of the total population.
As for the British banks, no one really knows how badly off they are, though the U.K.'s central bank gave assurance that it has raised more capital than they need. And while the balance sheets, preferred stock and bonds trade without much distress, the common stocks do. The British government own s 73 percent of the Royal Bank of Scotland, and 9.2 percent of Lloyds.
Since Friday, both stocks have collapsed and the government has lost 12 billion pounds on these two stocks alone.
With 12 billion pounds in immediate losses, that leads to a 4 billion pound deficit right out of the gate when compared to the 8 billion pounds saved annually from leaving. Thus, Cramer noted that if the U.K. had paid the 2 million immigrants 2,000 pounds to move somewhere else over the last decade — it would have cost only 4 billion pounds.
"In other words, even if you are one of the most xenophobic people in Britain, voting to leave the EU was the wrong way to tamp down on immigration," Cramer said.
There have been talks of a do-over for the Brexit vote, but Cramer doesn't think that is likely. Whoever takes over as the next Prime Minister will need to slow things down to let the world catch its breath in order for the pound not to fall farther.
As for other countries following Britain's footsteps, Cramer doubts any other country would make the same dumb mistake.
"The financial penalty for leaving the EU is now so palpable that only the most extreme of xenophobic nations would even debate the issue" Cramer said.
Ultimately, Cramer found this to be the dumbest financial mistake he could ever recall. This one decision has caused British taxpayers to lose far more money than they would save from leaving the EU.