Kim:
From searching around on the web it appears to me that if you pursue NAUI instructor training then you are obligated to buy text book materials from them that cost over US$1100. However I found an old thread on SB which said that this was not compulsory (this was a couple of years ago though).
Does anyone know for sure what the situation is with this now?
Each CD will establish the required list for his/her own ITC. There are some must haves (e.g., OW Instructor Guide) but there is a lot of variability in requirements across different CDs. For example, some candidates become ASHI (or Red Cross, or DAN, or...) instructors prior to coming to an ITC, and that removes a block of time and materials costs.
Another thing to keep in mind: NAUI instructors can teach a wide variety of specialties and leadership without having to re-qualify, pay extra, and go back into student/candidate mode after the ITC just to teach a specialty class. To prepare for this, the ITC requires candidates to teach master level / leadership / and nitrox lessons. It makes sense to include leadership and nitrox materials up front rather than hitting up a new instructor for nickels and dimes for the rest of their lives. It also helps you prepare to teach these classes if you're already familiar with the support materials your future students will use.
Like most things in life, there are different ways to think about costs. If you buy the Instructor Library at retail cost, the price is X. If you buy it at the CD's price, its Y (or Z, depending on the CD's purchasing level). If you buy it at the CD's price plus a couple of bucks, its yet another price. AND, if you only purchase the stuff required by the CD instead of the whole Library (minus the materials you already have) it is different still. Many CDs here will not charge full retail, and will pass along a savings as a way of helping you jump start your career as an instructor.
Pete's right - ya gotta get the CD to supply a list in advance. And be careful if you're working with somebody who isn't giving you info and expectations in advance! You don't need added stress at an ITC. Good luck!