I specialize in Part D, but have some familiarity with the medical side of Part C plans. The below is provided on the basis of general info, does not constitute specific advice, should not be taken as recruitment to sign up for a plan offered by the company I work for, blah blah blah.
In 2012 the standard Part B premium is $99.90. Some individuals may be subject to an income-related monthly adjustment amount causing a higher premium, though based on your OP I doubt your parents are subject to IRMAA. If your parents used standard Part B (aka Original Medicare) they would pay only the standard premium. They would also be subject to standard Part B pricing including the normal Part B deductible and coinsurance rates. Enrollment in a Medicare Advantage plan (aka Part C) carries a premium in additional to the regular Part B premium (for most plans, some plans do not charge an additional fee on top of Part B). The MA plans then provide a different benefit structure which can include a lower/no deductible, different copay structure, a maximum out of pocket, coverage of extra benefits like vision, dental, etc.
The plan I think your parents are enrolled in (based on info you provided in this thread and your prior one) does carry a premium that is in addition to the standard Part B premium, and this is very normal for a MA plan. If they are indeed enrolled in the one I think they are, then the price they are paying for the coverage itself is correct. It is also correct that the hospital would not bill Medicare directly - instead it would bill the health plan which would process all claims.
Three suggestions for your parents:
First, for the short-term to help sort out whether their hospital/medical bills are being properly submitted to their plan or not, and to help you and them better understand their plan/coverage, consider requesting help from the state State Health Insurance Assistance Program. For PA the SHIP is called
APPRISE. Their phone is 800-783-7067.
Second, as you mentioned your parents' resources are limited, it would be worthwhile to check if they are eligible for any assistance to reduce the cost of their coverage. A SHIP counselor at Apprise can review that with your parents, but you can also look at basic eligibility info
here. Even if you think your parents may be slightly over the limits, an application is still worth it as they may qualify.
Third, if their current plans are not working, they will have an opportunity to change plans during open enrollment. Open enrollment for Medicare Advantage is 10/15-12/7 and takes effect 1/1. There is an additional opportunity to disenroll from a MA plan 1/1-2/14 in order to enroll in Original Medicare (plus Part D if selected). Your parents don't have to be in the same plan, so if the current plan works for one but not both, one parent could make a change. Your parents may also qualify for a Special Enrollment Period to make a plan change outside Open Enrollment. This would be analogous to qualifying life events for plan changes that those of us in the working insured world deal with, just with slightly different criteria.
If you want to PM me directly regarding any of this, I'm happy to try to provide any more specific help possible.