Every member of NATO is required to spend at least 2% of its GDP on defense. Most don't. How much money does that shortfall come to?
I have taken the data from Member states of NATO and parse it. If I haven't made any mistake, which is entirely possibile, the Military expenditures of countries that spend less than 2% of their GDP is 158,786 US$ millions. If they all reached the target it would rise to 259,905 US$ millions.
The harder question is how much it would matter, especially in the context of NATO operations. Because it is obvious that even if Iceland would spend 2% of their GDP, they would hardly have the necessary resources to defend, for instance, Poland. If you look at more than mere numbers, only the US have the necessary structures and means to launch large scale operations outside their home country. The military intervention in Libya is a good example of this, France and UK really pushed for a NATO intervention but without the US they could have done little. The US government think that NATO could become more effective by achieving a greater integration and specialization of the individual forces.
It would be also interesting to understand why some countries do not meet the target. Among the big spenders only the US, UK and France spend more than 2% of their GDP, because they are the only countries that use military operations for political purposes. Spain, Canada, Italy and Germany are not directly threatened by any military enemy and have no interest in be involved in military missions. These European countries would prefer to have a no problem-open trade foreign policy with everybody.
Generally speaking the interest of the countries may be diverging, and since nobody has the power to influence the policy of the USA, one way to show opposition is simply to limit military expenditures.
All this to say that the apparently simple matter of the 2% target may hide larger political implications that would still impact the ability of the NATO to achieve their objectives as one group.
 now they spend 0.03%