Most estimates run in the 60-100 billion range.
I can tell you from being an owner of a large medical practice that billing services out is a tough and moving target. We have internal team of auditors to protect us, but it would be easy to code up here and there and very difficult to prove.
Of course... then you have the completely fraudulent and in your face addition of diagnoses by United Health and others that will never be tagged due to the $$$ in congressional coffers. And all of the outright fraud with medical devices, etc. The epicenter of much of that is in Florida.
It is a thorny and complex issue. Come down to hard on the innocent and you will decrease access to care and the grifters will retool and grift again.