This page contains links to very good but very cheap maths books on Amazon (in no particular order) - they were all available for less than £5 including postage when I wrote this!

Lorraine Mottershead: Sources of Mathematical Discovery 

Brian Bolt: Mathematical Activities

Brian Bolt: More Mathematical Activities

Stefan Hildebrandt: Mathematics and Optimal Form

Ross Honsberger: Ingenuity in Mathematics

H Steinhaus: Mathematical Snapshots

If you find any more please let me know!!