Arctic Cooling F12 PWMs are great, all-around fans that cost around $10. The only downside is that it is a very basic package; all you get is the fan, and four screws.
Enermax fans (Cluster, Cluster Advance, TB Silence, Magma) are also pretty good, though they are a little bit louder at full tilt (but, I think, are better for radiator/heatsink duty). They cost slightly more (~15) but most packages come with rubber isolators and a molex adapter.
Coolink SWiF2-120P are being sold on Amazon for $11-13. At full tilt (1700 RPM) the motor is still really quiet, since it has a true Fluid Dynamic Bearing, but the sound of the airflow will be noticeable. Not terrible, but noticeable. At around 1200 RPM and below you'd be rather hard-pressed to pick out the noise. They are a great investment, since they also come with rubber isolators and a PWM-splitter.
Cougar fans are ~$15, and are really good. However, the PWM variants often have poor quality components, so if you're actually using the PWM function, they make a higher-pitched whine that is annoying. You can bypass this by using voltage to control the fan speed, but that can be a pain (especially if the only way you can do that is with a hardware fan controller). However, Cougar fans also utilize a true fluid dynamic bearing, and I believe they come with with rubber isolators.
That should be a good enough list for you to research and pick a fan that is most suited to your needs. We can make further recommendations, but will require knowledge of what motherboard you have. (Case would be nice to, but not needed.)