Indeed it does seem cheap. They don't have to click on them, though. They don't have to log in and take out their CC to be effective. Neither do political TV ads and other older media. It wasn't enough to win the popular vote, but could it have been enough to change some folks' minds in swing states where the EC gives them more weight? Maybe yeah. Maybe enough that the feds are looking at the Facebook ad buys.
Fake news does not look like ads, and if targeted to receptive individuals, it gets retweeted/fb-shared to a like-minded audience. The comparison with "nobody clicks on ads" is not apt.