That’s James Pethokoukis’ take. His opening:

What would be the dollar cost of not bailing out Wall Street? Try a number north of $30 trillion. (The awful math is detailed below.) That’s why Hank Paulson and Ben Bernanke were so scared last week. And, yes, I think “scared” isn’t too strong a word. You don’t think they convened an emergency nighttime meeting of congressional leaders and then walked out with something close to a blank check for a trillion bucks because they thought we were headed for an outright recession, even a fairly nasty one?
Nope, I think they believed, and got Congress to believe, that the economy was on the verge of something far worse than the worst downturn in a generation. And that is why they went with the so-called nuclear option: the biggest financial bailout in history. In the words of JPMorgan Chase economist James Glassman, “Thankfully, we and our friends around the world who are watching the economic lights come on will never know where events would have led, if the clock had not stopped [last] Thursday afternoon…. Last week’s events made the 1987 stock market crash look like child’s play.”
As plumbers say about pricey repairs, “Sure, it costs money. It costs money because it saves you money.” And plumber in chief Paulson had a pretty big pipe, loaded with toxic debt, to unclog.
OK, let’s run the numbers. Paulson is asking for $700 billion. But that massive amount doesn’t include previous government actions to cure the credit crisis (like propping up Fannie Mae and Freddie Mac), nor does it take into account money the government may get back from selling the bad assets it will be purchasing. So let’s say those situations cancel each other out, and we are really talking about $700 billion. Now that money is being borrowed. So you take $700 billion borrowed for 30 years at prevailing interest rates, and you are talking about $2.5 trillion. But as Paulson said last week, “I am convinced that this bold approach will cost American families far less than the alternative: a continuing series of financial institution failures and frozen credit markets unable to fund economic expansion.”
Now let’s do the math on the “alternatives.” What would doing nothing cost?

He goes on to run the numbers.