I can't speak about medicine, but I know of a lot of dental practices where the normal fee schedule (list of fees for any given procedure) is higher than the fee schedule for insurance companies. As someone mentioned, the way a lot of insurance companies work (specifically HMOs and PPOs) is as follows: in participating with a given company, you (the doctor) agree to accept their fees, and agree not to offer procedures to their patients that aren't on the fee schedule (ie: if a patient needs a root canal, but the insurance doesn't cover root canals, you can't alert the patient to that fact). You then get what's called a capitation fee for every person who lists you as their primary provider (usually ~$2-3/month), and a co-pay from the patient whenever they come in (~$5-20). The capitation fee is supposed to cover "routine" procedures (cleanings every 6 months, etc.).
This system is supposed to discourage over-treatment. In reality, a lot of fee schedules are so ridiculously low (some fee schedules dictate lower prices than the doctor's materials costs or lab fees) that every time a patient who participates in that plan comes in, you lose money, thereby encouraging under-treatment and other (often unethical) cost-cutting methods. A lot of doctors deal with this by some combination of under-treating those patients and/or using cheaper, lower quality materials or cheaper labs that do poorer work (which is all unethical). Most doctors compensate for this by setting their standard fee schedule (ie: the fees that people without insurance pay) at higher prices.
This is all based on my dad's (who is a dentist) experience working for other people and in opening his own practice. Practices he's seen/worked in that accept a lot of HMOs have substantially higher fee schedules and tend to have cheaper, lower quality equipment than practices that don't. In his practice, he accepts a handful of insurance plans, a few PPOs, and no HMOs. His fee schedule is lower than a lot of others and he can do work to his own standards without worrying about losing money in the process.