In continuing with my awesome modeling tools for cheap, I'd like to share what I use for my general purpose hobby knife. Now, I like the X-Acto "Z11" blades, and I always have a sharp one on hand, but they're about $1.20 a piece. For my daily grind I use these scalpel blades.
The deal is you get 100 blades, AND a stainless steel handle--for $8.62 delivered. From the USA even! These blades are SHARP. They have a slightly different geometry than a typical hobby blade, and wear out a little faster when subjected to scraping seam lines and cutting hard plastic. That being said--they're about 8 cents each, so simply pop a new one in.
Being a cheap guy, I discovered that using a barber's strop greatly improves my blade life--for any hobby blade. After an hour of use, I just drag the blade back and forth over the strop, and it's as good as new.
Here's the shop I buy mine from. They usually arrive within 7 days. You can buy any selection of blade styles, and usually find one with that comes with a handle (I like the #7 handle and #11 blade myself).
And an example of their #22 blade on a #4 handle (I use this for sculpting foam), along with my strop.