They have thrown quite a bit of money at it, but in the year before the takeover Newcastle's revenue was approx £150m. That same year, Man City had revenues of more than £550m.
Bear in mind City even have a pretty low net transfer spend, because when you have a team worth £750-£1b then it's a lot easier to sell players and bring in new ones. If you have a squad worth £200m and want one in the same range as the current rich six, then you need to be spending MORE than the clubs above you.
Newcastle have spent as much money as they can, but they would need to be spending hundreds of millions, if not billions, to compete with the clubs at the very top.