I don't know much about NATO, and I hate Trump, so I came prepared to see this argument as yet another reason to hate him. Keep in mind this man said that NATO was obsolete when interviewed during the campaign, then later admitted that he said that because he didn't know much about NATO: i.e., Trump admitted to talking out of his ass about NATO. However, after reading a few articles and the wikipedia entry on NATO, I can't add this to the list.
My understanding is that the member nations of NATO pay the agreed amounts for direct costs (how much it actually takes to run NATO's military and non-military programs) following some formula that takes into account GDP, etc: the US ends up paying about 22% and Germany is next with about 15%. Now I believe the US's GDP per capita is larger than the entire EU, so it seems reasonable that we pay 1/5 or so of the ongoing costs of NATO. Trump and others may disagree, which is fine as long as you have a convincing argument for why this formula isn't fair and a better way of determining the share we should pay, then we need to renegotiate the treaty. No, flat costs don't make sense: Slovenia cannot afford to pay as much as the US … The indirect costs seem to be the major bone of contention. The member states all agreed to spend 2% of their GDP on defense. The US currently exceeds that at 3+%, while only five of the other states (of 28 total, I think) have met the goal.
TBH, i don't know what the point of NATO is. We don't need a huge military alliance to deal with one piece of shit country (Russia), so there must be more to it. I'm still going through the arguments pro and con.
Take away: Trump is still a dung beetle, but it's true most of our NATO allies are not meeting their commitments.