I don't know about everyone else but I know that, while I love bookbinding, it can be hard to do it when the materials can be so expensive, so when I want to make books but don't have the money for supplies, I go looking around to see what I have that I can use. For example, I used a New York City metro map for the signatures in this book. Not really a book for writing in, but still a fun book. According to Natalie, there's a Mr. Grape, who has an office on the second floor of the library (I think) who will give bookbinding students old maps for free. Something to look into...even though I haven't.
Another way is to use old paper from art projects or other books. I found that I had a bunch of glassine from when I was in printmaking classes and so I used it in a book for either tracing or keeping pieces in to protect them.
More map use. Bus and Subway maps can look really cool and can usually be found for free.
These are just a few ideas on reusing things, I'm sure there are many more out there and better ones as well.