A big deterrent for me entering the eBook market was the high price tag on eBooks and when Amazon brought pricing down, I jumped in. I didn't expect to spend less on books; however, I didn't want to spend more.
With pbooks, I never paid full retail (using Coupons/buy one get second 1/2 price or Amazon) and probably bought 50% used. Much to my surprise, I have saved considerably over what I used to spend on books (and am reading much more):
1) Less impluse buying - I was always buying books just to have them for when I wanted to read. Many piled up in boxes as I bought more books. Sometimes I would have two copies of the same book. Now, I mostly buy when I am ready to read.
2) Samples - I often bought books only to find they weren't what I expected.
3) Public library - although its not a large percentage of my reading due to limited titles, it does supplement what I buy quite nicely.
4) Free/cheap downloads - although not as much as some others as I don't read a lot of the genres that seem to have the most offerings (horror, sci-fi/fantasy, etc.).
Granted the saving aren't the same for everyone...I am sort of a book store junkie. It wasn't unusual for me to spend $50-$100 per visit to the brick-and-mortar stores and always had books coming in the mail via Amazon or the marketplace.
Conservatively, I would say that my Kindle 1 will pay for itself in 10 months or less in book savings alone (a year if you count the extended warranty and third-party cover/light)....as long as I can resist the desire to buy a newer reader and/or more accessories