TomTop offers the Bayasolo X1 In-Ear Headphones for $5.99. Coupon code "PAE0088WCX" cuts that to $2.99. With free shipping, that's $10 off list and the lowest price we could find. They feature a frequency response of 20Hz to 20kHz, microphone, and in-line controls.

Note: These ship from China and may take up to four weeks to arrive.

Update: The price has dropped to $2.99.