Amazon offers Prime members the Phanteks 140mm CPU Fan, model no. PH-F140HP_BK2, for $14.99 with free shipping. (Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $8.77.) That's $5 off list and the lowest total price we could find, outside of the deal below. It adjusts from 500 to 1,600 rpm.
A close price: Newegg offers it for $14.39 via "MAYPHANTEKSPREF" plus 99 cents for shipping.