if you already have a controller for PS2 (and you're comfortable with using it) and you aren't looking for any extra features, getting a simple adapter for PS2 to USB will run you about $20 (maybe cheaper today). With that you will however have to configure the buttons and joysticks.
xbox wired controller with go for ~$40-50, but as said above, many games support the controller and automatically recognize it or have prebuilt configuration for it when you switch input to controller from keyboard (especially in games that are console ports to pc)
if you're looking for some extra features (programmable buttons, turbo, etc) you need to explain that as you'd be looking at 3rd party controllers for that.
i use the xbox 360 wireless controller. i got it off amazon for $45, works great, ive only run into two games that didnt automatically detect it. MW3 and payday. You can download xpadder though and it will fix it.