I'd hate to use a Spyderco as a beater. I assume you want the knife as a dirty work/loaner back up to your larger, more expensive folder. Like, if you need to scrape something or someone wants to borrow a knife to cut sandpaper or something stupid like that, you have the cheap beater so you're not abusing your "real" knife. Am I right?
For cheap, light, and small, there's the Gerber LST. Probably half of the people here have had one or five of those in our time. $10-20 depenging on size. The smallest one at $10 is a good value if it's planning on getting abused. Buy a few.
Honestly though, the Kabar Dozier is just about the best bang-for-the-buck knife out there. Too large though?
The Spyderco Jester or Ladybug make great small knives, but at $20, they're not quite the throwaway beater kind of knife.