Take look for tutorial information at SparkFun and Adafruit on Force Sensitive Resistors. I have not seen an example of it used for drum pads but it might work.
http://www.sensorfilmkit.com/6"x4" sensor material for $18. If you cut them into 1" squares, 24 sensors for $18.
I have not used the sensor film yet. There is a lot of information on their website. For questions about the film, I suggest contacting the company via their website Contact tab.