So far, the most expensive book in my collection set me back $250.
Maybe some day I'll be able to purchase things like this too. Drooool.

Capek, Karel. R U R ROSSUM'S UNIVERSAL ROBOTS: KOLEKTIVNI DRAMA... [Praha]: Aventinum, 1920. Octavo, pp. [1-6] 7-96 [97] [98: blank] [99: colophon] [100: blank], original decorated lavender wrappers, printed in brown and black, all edges untrimmed. $12500

First edition. 2000 copies printed. Signed by Capek on recto of first leaf.