great books for not much money
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!!