A thermophile is an organism that lives in hot conditions, like the hot springs at Yosemite.
A thermopile might be found in an IR thermometer.http://www.mouser.com/Search/Refine.aspx?Keyword=thermopile
Looks like $4.55 for 1 with a 10x10 array 'only' $358! Or maybe a 100x1 'wand' to make a low-res raster-sweep with?
One app I see is to know if some part or area of your circuit or room or factory is getting warm.
A PIR has a special dual pyroelectric sensor with lens and filter to work as a motion detector for mainly humans. It works by detecting the difference in sensed temperature between 2 sensors that look at different areas. I can't but wonder however if the lens and filter were removed, or just the lens removed and then one side masked if it couldn't be used as a straight IR sensor? Or perhaps the thing would either be too sensitive or pushed to some limit?
For just the sensor at Mouser, I see prices on some less than $2.