The last time that I spent much time researching an inexpensive/throw away self-defense knife, I recommended a Cold Steel Kobun -- they can be had for about $30 from Amazon. In addition to price, requirements were that it couldn't be a dagger or "Bowie" and it had to be concealable.

When I was in high school, I carried a comb that had come with a handle in my back pocket (it was the 70's, so that wasn't as uncommon as it would be now). I removed the handle and substituted an icepick. One day a guy tried to pull a pocketknife on me and I had the point under his chin before he'd opened his blade, so he put it away and moved along. An ice pick with a "sheath" like this: http://www.amazon.com/Harold-Stainless-Steel-Covered-Pick/dp/B0001V489W/ might be worth considering.