I'm reading all these replies and wondering if Carbon is feeling as confused as I am.
Seriously, though, "confused" is pretty much the typical state of mind when using the US health care system, and what Frank said about asking the cost of every single service is excellent advice.
If you really think he needs to have an official sleep study, Carbon, then I'd agree with the person who suggested calling around and asking for a cash price.
He can check with his insurance provider to be sure, but my experience with health insurance has been that the contracted insurance rate doesn't apply to anything until after you've met the deductible.
If he does decide to do a formal sleep study, be sure to find out how many doctor's visits will be required along with it (I'd think one before and one after the study) and if those will be discounted as well. If they aren't, an initial visit to a specialist is often $250-$350, and the follow-up to get the results of the study is also a longer (more expensive) visit than typical doctor's visits.
Going the sleep study route will give him access to a sleep doctor which can be helpful. (I had two sleep studies and they were billed at around $1,000 each, which was actually much cheaper than I would have expected.)
The least expensive and simplest route would be to get a primary care doctor or dentist to write a prescription and just buy an APAP and mask online and do his own titration. That will leave him without access to a sleep specialist, though, and he also won't have a respiratory therapist at a DME to help him with mask fitting.
Oh - I have seen on some of the online suppliers' pages that they sell home sleep studies that a doctor (or facsimile) will review and then write a prescription for the machine.