Muggles had a lot of gold a hundred years ago, too. Certainly if you count in terms of just a few wizards taking possession of it. And it was easier to find because currencies were gold-backed.

If they were unwilling to just steal it (for whatever reason), then spending a few weeks performing services for the richest people in the world in exchange for half their wealth (amounting to many millions of Galleons) would have been a great bargain for wizards.