Detecting human presence - LMI 878 IR detector

I ended up getting a panasonic pir sensor that has everything integrated into the tiny lens package. It works perfectly but im a little bummed they are $10 each. I was going to look into building it myself with just the sensor, lens, and try to make or buy the board myself for cheaper but I don't know how much that would save me in the long run. I might just deal with the price tag for this massive amount of convinience.