Cougar Turbines are $30 for 4 , so that's about $8 each http://www.newegg.com/Product/Product.aspx?Item=N82E16835553009
Discounted Cougar Vortex HDB (300K hour one) go for about $8 , right now $9 if you can stand orange fans
$16 - $6 off w/ promo code EMCXMXM76, ends 8/14 http://www.newegg.com/Product/Product.aspx?Item=N82E16835553001
I wouldn't buy the fan separate from the heatsink though, when you can buy a Thermalright True 120M for about $30 or Thermalright True 140/HR-02 Macho for ~$40.
Arctic F12s are $8 http://www.superbiiz.com/detail.php?name=CA-F12
$9+shipping Arctic F12 PWM with cables sleeved http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=36_407_696&products_id=26686
Arctic F14 $9+shipping http://www.superbiiz.com/detail.php?name=CA-ACF14
Scythe/Nidec Gentle Typhoons go for about $10-12 but not anymore. http://www.sidewindercomputers.com/scge120mmsic2.html
^ best option for radiator
Thermalright TY-140s and TY-147s sometimes go for about $12 http://www.superbiiz.com/detail.php?name=FAN-TY147
Gelid Silent 12 PWM 1500RPM max http://www.superbiiz.com/detail.php?name=FAN-PX12
(not as good as Gentle typhoon/Cougars/Arctic on heatsinks)
lower RPM fan Gelid Silent 12 only meant for Case fan http://www.superbiiz.com/detail.php?name=FAN-FN12SX
Given that the price of generic Yate Loons (see http://www.performance-pcs.com/catalog/index.php?main_page=index&cPath=36_1130_49_1051
) or other fans is the same as the Arctic F12s I'd go with the Arctic F12 if you hate orange
Keep in mind if your motherboard only supports PWM speed control for the CPU fan header you'll be annoyed by PWM tick at lower RPMs unless you grab Noctuas, Be Quiet, Thermalright, etc that are not known to tick.
Any cheaper and you're looking at stuff like rebated $3 after rebate Cooler Master Sickleflows (http://www.newegg.com/Product/Product.aspx?Item=N82E16835103061
) which aren't that great and have sleeve bearings.
FrozenCPU also has many fans, but the cheapest is $6-8+shipping (Yate Loon/XSPC ones that are sleeve bearing)
140mm : http://www.frozencpu.com/cat/l3/g36/c15/s775/list/p1/Fans-12_Volt_Fans-140mm_x_25mm_Fans-Page1.html
P.S. you need to be more specific
How important is noise? What is your tolerance in dB? High frequency (ringing/coil whine)/low freq (hum/mechanical bearing noise)?
What motherboard will you use (or fan controller)? Do you need PWM to control fan speed?
How far are you planning to OC?
What heatsink will you use? How willit be oriented (if you face it up then you can't really use sleeve bearing fans)
How good is your case airflow?
How much is the budget ideally? etc.
mobo, RAM, case
Any color (Noctua brown & cougar orange included) or do you have a windowed case and want to stick to a color theme (blue/white/black)? Some people are specific , i.e. "I want red LEDs".
Some resources for you
http://www.silentpcreview.com/section12.htmlEdited by AlphaC - 8/13/13 at 7:20pm