Well this is only $9.00


"Infrared reflective light sensor for Handy Cricket. Works best at a distance of about 0.4" (1 cm) from a surface. Transmit a small beam of invisible infrared light and measures the amount of light that is reflected back into the sensor. Also is sensitive to ambient light; to measure just reflectivity, shield the sensor from ambient light."

Works at about 3/8 inch from the surface it's detecting, sounds like it could work.

I found it at http://www.gleasonresearch.com/prod.php?sku=CX-IR-REFL