You can get knives at all price ranges and there is some really awesome stuff in the $30 range. If you want the Surefire of knives, then we are talking anywhere from $100 to several hundred.
You mention you want a single blade, but you would also like more functions.... So my question is, how do you plan to use the knife? Do you want something that just has a good blade for cutting anything? Or are you wanting something that has screwdriver blades, can openers and other various tools?
You say you want keychain size or maybe a little larger. The good news is, there is a lot to choose from in that size.
Another thing to consider, is for the blade of this knife, would you like a plain smooth blade, or do you want serrations? You can also get blades with half smooth and half serrated. Serrations are really good for cutting lots of things, but are especially good on rope and cardboard.
My EDC as far as knives go is a tactical folder(lately it has been one of my two CRKT Crawford/Kasper's) with a pocket clip and a blade of 3 to 4" Then I carry a Mini Buck on my keychain, it has a 1 7/8" blade. And on my belt next to my SF, I carry a Gerber Multi-Plier 600.
Swiss Army knives are by no means on the level with Mag. As long as you stick to the two main brands(Vitorinox and Wenger) you will be fine.
Here's a suggestion that may be exactly what you are looking for, the Leatherman Micra
http://www.leatherman.com/products/tools/micra/default.asp
I have given a couple of these as gifts and they are always very appreciated.
--Jason