I personally know > a dozen guys from my old company who interviewed at Airways and were hired without any sort of professional interview prep.
Hell the only thing I bought for my interview was a belt, and I returned that the next day
. I did have my logbook bound at Staples though after printing it off at home.
IMO it's way overblown depending on the company. For the carriers that require a sim eval I would probably do prep on that just so you feel more comfortable.
I guess if it puts you more at ease it's worth the $$.
Prior to my interview I did call up some buds who were successful hires at another legacy who had done Emerald Coast prep, and they pretty much told me what was in the book "Checklist for Success" by Cheryl Cage.