I took a course with a company in VA and after a week of nonsense I realized- and so did the other 14 others in the course [of whom only 2 have actually become certified]- 2 things.
1) What a HUGE waste of money spent for the "knowledge" to pass the exam; we could have spent 1/10 of the amount...actually I could have spent a much lower portion because I took the week long course far from home and had to get hotel/food for a week.
AND 2) Yes- soup to nuts- the whole thing is sketchy and is basically a lobbying group to congress, but can be a good route for the right person [who has the capitol and existing business]. Assuming you pass the exam the first time [we were told by one of the RESNET board members only 65% do] you then have to pay a "provider", essentially someone to review your first several audits; regardless of what they say, this is a real money maker. They charge several hundred just to take you on and then you have to pay for each audit they review and most of the time you need to purchase your own equipment [we're talking minimum 5k for the equipment and more like 10K and up]. After a certain amount of audits [I think offhand it's 5] you officially become certified. And once you do get certified and want to continue to do work you need to pay a provider to continue to do review audits b/c in order to maintain certification you need a certain percentage reviewed each yr. Crazy.
All of that said, the knowledge of building science is a great thing to have. In this economy being able to put one more thing on the resume is a good thing and for less than $100 [assuming you pass the first go around] you can say you passed the exam and are on your way to certification.
This is also not a bad gig if you want to start a rater business or couple it with your existing business [contractors, auditors, home inspections to name a few]. Again, it's not a bad thing to consider because there is money in it as long as you have the capitol for the equipment and perform enough ratings to justify the expense of having a provider review your audits.
In either instance what I'd do is NOT
TAKE AN ONLINE OR IN-PERSON COURSE simply because of the absurd amount they charge and what you get in return. Instead, purchase some of the cost friendly materials out there and pay the $50 to take the exam- even if it takes a couple of whacks, it's still cheaper than those $1000+ courses. There's some study material/practice exams on eBay and I can't speak to those but the one I used for my exam [I passed the first time] was at http://hersrater.blogspot.com/