NYT:

In an unprecedented effort to self-finance a presidential campaign — which some rivals derided as an attempt to buy the White House — Mr. Bloomberg’s bid cost him more than half a billion dollars in advertising alone. He also spent lavishly on robust on-the-ground operations, with more than 200 field offices across the country and thousands of paid staff. His operation dwarfed those of Democratic rivals who ultimately won states in which he had installed many dozens of employees and spent heavily on radio, television and direct mail ads.