-3d accelerometer -39.99 http://www.parallax.com/Store/Sensors/AccelerationTilt/tabid/172/CategoryID/47/List/0/Level/a/ProductID/97/Default.aspx?SortField=ProductName%2cProductName

-light sensor – 5.99 -> 4.79
http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/Level/a/ProductID/88/Default.aspx?SortField=ProductName%2cProductName

-photoresistor – 1.99 -> 1.59
http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/Level/a/ProductID/175/Default.aspx?SortField=ProductName%2cProductName

-infrared sensor – 3.95 -> 3.16
http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/Level/a/ProductID/177/Default.aspx?SortField=ProductName%2cProductName

-compass – 29.99 -> 23.99
http://www.parallax.com/Store/Sensors/CompassGPS/tabid/173/CategoryID/48/List/0/Level/a/ProductID/98/Default.aspx?SortField=ProductName%2cProductName

-thermometer / humidity – 35.99 -> 28.79
http://www.parallax.com/Store/Sensors/TemperatureHumidity/tabid/174/CategoryID/49/List/0/Level/a/ProductID/94/Default.aspx?SortField=ProductName%2cProductName

-4 hall effect sensors around the rims – 4?

Parts for 20:
122 if from parallax 1 by one
106 if bought 20 at a time from parallax