If you aren't obsessed with exactly matching a particular beastie from the Monster Manual, there are a lot of inexpensive options out there. I've never felt obligated to use the miniatures that game companies make for my gaming when a good toy or craft store can provide me with some pretty good monsters.

Michael's, in recent months, has been carrying a NIFTY line of inexpensive-but-well-sculpted plastic dragons, but even the Dollar Store can provide some fun and useful Monster Minis.