For a non-fiction book like yours at that very expensive (for an e-book) price point, yes, at minimum, you should have at least 1000 email list subscribers.
Honest question, and I'm not trying to be combative, just seeking informed feedback from those who have done it before: have you written and successfully marketed an e-book before? Why is having 1000 subscribers the minimum, based on your experience?
FWIW, I tested a range of price points for both the book and the bonus content through surveys of dozens of graduate students at R1 research institutions, and my price reflects the 25th percentile of prices that people said they'd be willing to pay based on the details I provided them about what the content of the book and bonus content would be. I found that students attending these schools typically spend at least $1,500 on the application process (test prep/guidance, standardized testing, and applications to schools (avg. applications sent out ~8) themselves).
CollegeStache:
Yes, I have published 3 romance novels and a number of short erotic stories, and have been doing this for three plus years. I do this part-time, but I have a number of friends that I communicate with who do this full time, started at the same time I did, and are making in the mid six-figures per year writing e-books. I also have a number of friends that I have met through this process who publish non-fiction, and I am basing my remarks on those experiences.
You may well be right about the price given your research, but I'm not sure. My understanding was that your book was to be marketed to a general audience, not students. Is this correct? If so, your research might have been skewed by the fact that you were asking graduate students in an academic context -- in other words, you are asking people who regularly spend lots of money on books. If that is your audience, then that's great, but I'm not sure it is.
As I said, non-fiction is not my field, but I might suggest, if you haven't done so already, that you read some books by successful non-fiction authors that talk about price points, etc. Again, I'm not sure who your market is, nor through which venues you are planning to sell this book, but I have never run across an e-book on Amazon that is this expensive. Granted, given that you are planning to also sell premium online content with this book, my guess is that you are planning to sell it on your own website. In which case, I think it's even more important that you have a large email list, because you will not have people who randomly come across your book in an Amazon search.