Price is a factor because so many of us are hardcore bookaholics.
To someone who buys two or three books a year, a $10 difference in price is no big deal. It gets lost in the noise. But to someone who, on occasion, buys two or three books a week, it's a very big deal. The serious readers among us buy a lot of books. I know for me, personally, the two things that have constrained me are price and shelf space. Ebooks transcend shelf space, so that brings price to the forefront.
I think you might want to break down the poll into two categories: positive factors that make people more likely to buy a book (good reviews, for instance) and negative factors that make them less likely to buy it (priced higher than a pbook). Any purchase decision is a tug-of-war between the plus and minus factors, and it's really hard to judge it without taking both into account separately.
For me, for ebooks, DRM is an absolute deal-killer. For pbooks, it's form factor: I almost never buy hardcover fiction because I haven't anywhere left to put it. That's not a problem that the person who buys two or three books a year has to worry about; for me, it's a matter of not having to sleep on in the back yard because the books have taken over my bed. So you may be polling two separate groups of people (the casual readers and the hardcore readers) and getting results about as valid as if you polled casual and hardcore World of Warcraft players on their interest in game features.