Spouse of a third year resident here. My wife has volunteered at several primary care (i.e. generic doctor) clinics in Manhattan and nearby area (Queens, Astoria, Brooklyn and even in Boundbrook, NJ). These are all owned by the doctors. She worked in all kinds of operations of a clinic from billing, sending prescription, taking patient history, etc.

Back-of-the-napkin calculation based on my wife's experience -- if you are a primary care (generic) doctor in a highly populated area and you see ~40 patients a day and each patient pays $100/visit.

$100 * 40 patients * 22 days per month of work * 12 months = $1056000 (about $1 million USD)

We are using $100/patient/visit and that's being very conservative. Most of these doctors would encourage the staff/volunteers who do the billing to put as many relevant billing codes as possible in the system (e.g., if someone comes in with a cough, they'll try to bill for anything related to cough symptom although they already knew it's just for seasonal allergy). This is necessary also because a lot (not all) of the patients are on medicare/aid and the reimbursement from medicare/aid is not as good as the ones from the private insurance companies. The cost of the labor is mostly just one assistant or at most two for ~$20-$30/hour max. Not sure about the insurance cost (but we assume that it would cost ~$20-$30K/year for malpractice insurance?) and renting the clinic (some docs do own the clinic). All in all, we believe that having your own practice can net you a lot more than what you'd make by working as an attending/hospitalist at a hospital (on average, hospitalists make between $250K-$300K/year, which is still commendable).

But if you live in a rural area, which tend to have smaller population density, then you are probably better off working for a hospital because as a hospitalist, you can make close or a little more than $300K/year as a general doctor. My wife knows a couple of doctors in Palmdale, CA, who work as hospitalists in two hospitals (6 days a week alternating between two hospitals; the days start at around 9am and ends around 3-4pm). They rake in ~$600K/year from salaries (not including bonuses). A friend of mine just completed his residency and got an offer from a hospital near Dyersburg (Tennessee) for $300K/year salary with $40K sign-on bonus. The specialists earn more of course: https://www.whitecoatinvestor.com/how-much-do-doctors-make/. A cardiologist friend of mine told me that he got an offer from a hospital in Montana that pays $800K+/year.

Just wanted to share what I know about how much the doctors can make in the United States.