I'm no fan of Trump, I actually think he's a bit of a loose cannon.
But, He does have a bit of a point - many NATO nations don't pay their way, they do freeload on the back of the US (and the UK for that matter since they know that we [US, France and UK] always can be expected to punch above our weight - rightly or wrongly)
But, He needs to decide if that is a price worth paying to maintain the profile of the US overseas and maintain the free markets that the US sells so much hardware into.................. I expect the cost of the NATO 'subsidy' is a fraction of the cost of the equipment sold to NATO partners by US Defence contractors which ultimately puts $ into the US Treasury through the tax system and keeps a lot of people at home employed and fed................. he'll certainly understand that aspect.
Me, I think he'll recognize the commercial aspect pretty quickly.......... what he does on things such as the economy, climate change and world trade agreements is a whole other question though.
Arc