Although I believe that construction of the Clutch airfields preceded Germany's joining NATO, they would almost certainly have been financed through the very early 'Slices' of what was then the NATO Infrastructure Programme - ie by cost sharing amongst the NATO Members of the time. I am unclear as to how Germany stood legally as "Host Nation" before becoming a NATO ally in 1955 but, on joining, it took on a substantial share of all Infra costs, largely on account of the number of facilities seen as being militarily required there.