If you're looking for a good LAV Mic, go for the Rode LAV which runs $249 dollars. Doesn't require very much gain, and sounds very clean. The Rode allows you to use an exclusive MiCon adapter that can be swapped to support basic 3.5mm, XLR, and more.
Rode LAV Mic with MiCon Connector
Ok so $250 for a LAV is a hike for many, so on the other end of the spectrum is a few inexpensive LAV mics. One you might have heard about is the Audio Technica ATR3350. The sound is not to be compared to a $250 lav mic, but as long as you have a decent portable audio recorder with enough gain, it works in a pinch. We use this $17 dollar LAV mic quite a bit mainly because of the very very long cord that can be run from the the talent all the way back to the Camera Operator without using a wireless setup.
Audio Technica ATR3350 Lav Microphone
Another cheap LAV with a 4 star review is the Pearstone OLM-10 Omnidirectional Lavalier Microphone. It's also designed with a very long cord that can be worn by the talent and run to the camera a good distance away. Currently it's on sale for the next few days at just over $16 dollars via B&H (click here)
Pearstone OLM-10 Omnidirectional Lavalier Microphone
Another budget LAV mic shared by J Hanna in the comments is the Olympus ME-15. This one does not require batteries like the other two above, and comes with a 3.3ft short cable. You can find this for just over $20 bucks (click here)
Olympus ME-15-LAV Microphone
[Updated Entry 2014]
Recently my new favorite affordable LAV Mic is the Aspen Mic series. These LAV mics require very little gain from your recorder so the sound quality is very comparable to high end lav mics. Priced under $45 dollars, it's by far the best of this lineup if you can't afford the $250 Rode LAV. Check out the Aspen Mics website for their products (click here).
Aspen Lavalier Microphones Mono / Stereo / Sennheiser G3