There is about 2.5 billion euros in budget used by the organization directly. This includes people employed directly by the organization, costs related to head headquarter buildings, etc, some command and control infrastructure owned by the organization rather than individual members, some things like deployable radar systems owned by the org directly, plus funding for certain operations that members have agreed to fund via cost sharing, rather than by having any participating members finance their own costs.
Remember, NATO is not just a defensive pact, but also command structure set up to enable the member forces to be utilized as a single cohesive force, rather than just a bunch of allied but disjointed forces. That means communication systems, and setting up unified commands to which portion of member forces can be assigned, making sure equipment and communications systems are intercompatible, etc.
This central funding money is pretty negligible compared to the 2% spending on defense commitment of members. Like the whole central funding would be around 6% of Germany's military budget, but would be fraction of a percent of the US budget. However the central funding is shared amongst all the members. Germany's actual share ends up around 1% of its military budget, UK's actually share is about 0.6% of the UK's budget, and the US share might as well be a rounding error, being my my quick calculations less than a tenth of a percent of the overall budget.
It's certainly not nothing, but is is not like we are talking about super significant portions of countries military budgets.