Woah, what a cool find! My coolest old book finds are a hardcover binding of half the year of National Geographics from some year in the 20s (it, like your book, is in a tub somewhere in my mom's attic; one day we plan on tackling her attic and I shall find my missing box of books, lol), old copies of Love's Labors Lost and The Comedies of William Shakespeare, and two world atlases, one from during WWII so the maps clearly show axis/allies lines! One of the atlases also has several b&w photos of Chicago in it from the 30s I think was the date on the back of one.
I haven't paid that much for a book, though, I don't have that kind of fun money. XD The most expensive was the National Geographic collection, which was $20. The Shakespeares were under $10 each, and the atlases were part of a "buy a whole bag of books for $1" at the Salvation Army store years ago. :D
I haven't done thrift store books since then, though - because I brought home bugs in one once. I had bought an old bible that was just very cool and put it on the shelf; when I went back to it, it was just COVERED in so much bug ick I threw it away entirely. It took some work to clean off the other books on my shelf, which were thankfully not that hard hit yet. I was mad, though!