While the current consensus among American dentists is that flossing helps prevent cavities, this (like what seems to be a frightening amount of what dentists believe about dentistry) is not actually founded on or backed up by scientific evidence. There is slightly stronger evidence that flossing helps against gingivitis (http://www.ncbi.nlm.nih.gov/pubmed/22161438).

As for dentists handing you a bill that would pay for a used car? There's an easy fix for that: ignore most of what they tell you, and get things fixed if/when they hurt. When the dentist has you sign a consent form that says "dentistry is not an exact science", take heed.