The Etymotic Research MC2, the headset version of the dynamic-driver MC5 model, is currently available for $19.99 + shipping from amazon.com (sold by 3rd party, so not prime eligible). The MC2 typically runs $80-100.
The MC2 features a microphone and single-button remote and is compatible with Apple and most Android/Windows Mobile devices. Otherwise it is identical to the MC5 model, which I reviewed here. This is the lowest price we’ve seen on any variant of the MC5, making it less expensive than Etymotic’s entry-level earphones, the Ety-Kids EK5, and a highly recommended buy for clear, detailed sound.
Edit 01/11/14: now expired