Really just depends on what you want to use it for. I have folders that I use for anything and everything and then I have folders that I wear when I have to dress up and go out with the wife that I use for only clean cutting and small cuts...then I have folders I carry with all intentions on using as much as I can to see if I can wear it out....I even throw a few cheapies in my pocket to give away sometimes when I'm going to a outdoor activities where I know people will be asking to use my knife.
A good start is spyderco....you can get some decent folders for under $100 and they will hold up nice.
If cheap is your goal...then buck makes a few folders that will stand the test of time...
It's an addiction, you'll start buying knives from every corner of the internet....lol.
I'll second the Spyderco's, however beware that not all Spyderco's are the same. All of the one's I have are good but the one's made in Golden, CO are by far the best of the bunch. More expensive than the one's from China but better steel and quality all around. The PM2 is a good example.