Finetech007 via eBay continues to offer this Mini Stereo Microphone for Smartphones for $2.67 with free shipping. That's tied with our mention from last week as the lowest total price we've seen. (It's a current low for such a microphone by $4.) It connects via a 3.5mm jack and records volumes up to 110dB. The microphone features a tilt-up mechanism to reduce contact interference.
Note: This item ships from mainland China and may take three to eight weeks to arrive.