Krugman tallies it:
If you add up the lost value since the slump began, it comes to some $3 trillion. Given the economy's continuing weakness, that number is set to get a lot bigger. At this point we'll be very lucky if we get away with a cumulative output loss of "only" $5 trillion. These aren't paper losses like the wealth lost when the dot-com or housing bubble collapsed, wealth that was never real in the first place. We're talking here about valuable products that could and should have been manufactured but weren't, wages and profits that could and should have been earned but never materialized. And that's $5 trillion, or $7 trillion, or maybe even more that we'll never get back. The economy will eventually recover, one hopes–but that will, at best, mean getting back to its old trend line, not making up for all the years it spent below that trend line.
I say "at best" advisedly, because there are good reasons to believe that the prolonged weakness of the economy will take a toll on its long-run potential.
The Dish 