Most of the better knife brands (Benchmade, Emerson, Al Mar etc) have a few models that run slightly under $100. Columbia River makes solid knives for $50 to $75. Any of those should serve you well. Buy the one that feels best in your hand.
Online mail order stores can usually save you a bit of money over paying retail at a real-world store. The problem is that you can't hold a knife in your hand if you're shopping over the internet. A knife is a hand tool, and it
must fit well in your hand.
One feature that your knife oughta have is a lifetime warranty. If you buy one you won't need it; if you don't then you'll probably wish you had someday.
Serrated edges are overrated. Buy a straght edge, and keep it properly sharpened; it'll cut anything a pocket knife has a right to cut. Avoid the black-coated blades, too. After a little use, most coatings will eventually scratch, chip, peel, and so forth, and will leave your knife looking ugly.
Consider buying a fixed blade knife instead. $100 buys a superb fixed blade knife, compared to a mediocre folder. Plus, fixed blades are inherently better than folders, even if the quality is comparable. Folders are more convenient, tho...
I really like knives. Lemme know if I can answer any more questions for you.