FWIW, part of the reason you see such a huge spread in costs is that providers are repeatedly warned never to discuss fees with each other because of antitrust laws. (Don't get me started on how managed care "negotiates" (= dictates) fees because they are exempt from those same laws, grrrr.) I can tell you that my fees are within what is being discussed as typical for MA and/or for someone who is expert with working with GT and possibly-2E folks. I'm not cheap, but I like to think that the value I provide to clients is worth it.
In general, most insurance will not pay for achievement testing at all -- they say that schools should do that for free (of course, the schools tend not to agree, leaving the parents in the lurch).
In general, also, insurance tends not to want to pay for something unless there is a pretty clear case to be made that there's a Big Problem Worthy of Psychiatric Diagnosis. "Oh, we just want to rule out ADHD," will often meet with the response, "Okay, so just do a questionnaire for the parents and the teachers and ask them if they think the kid shows symptoms of ADHD." I had a kid I filled out insurance forms for at my internship (I had to make a strong case literally test by test), kid was obviously seriously depressed, and they said, "So just send him to a psychiatrist and give him antidepressants, he doesn't need testing."
Note that many private providers don't take insurance, because the reimbursements are so low (and getting lower all the time).
Note also that school-based testing is answering a different question from private testing. School-based testing tends to be about the numbers, not the collection of rich data and interpreting it in a thoughtful way. So it depends on what kind of questions you are asking and who your intended audience is.
University-based testing can be very hit-and-miss, as others have noted. You might get someone who is only in the clinic because they have to fulfill a requirement, and you're unlikely to get anyone who has specialized knowledge about giftedness... or you might luck out and get someone terrific with a fantastic supervisor and get a gold-plated comprehensive evaluation for pennies on the dollar. (After all, I was an intern in my school's clinic last year (grin), taking on the most complex cases I could.)
If you only want to know, "Does my kid have a 145 IQ and achievement so he can qualify for DYS?" then one could argue that if you could convince the school to test for free, there would be little reason not to take that route. You wouldn't know much more about your kid when you were done than you did already (you probably already knew he was smart because that's why you were testing), but sometimes that's all people actually care about. I'm not shy about telling people when they can get something cheaper someplace else. Most folks who work with me or someone like me are looking for more of a comprehensive and thoughtful analysis, with a question more like, "What is my kid like as a learner, what would be the best ways to help him learn, where would be some good directions to go next?" or even, "If my kid is so smart, why on earth can he not do <whatever the problem is>, and what can we do to help him?"
