For Christmas I built my brother a heated hoody it ended up costing around $100 but it is pretty badass.
4x 5x15cm Heating Pad @ $4.95 = $19.80
https://www.sparkfun.com/products/11289
1x Wearable Keypad @ $12.95
https://www.sparkfun.com/products/10411
8x 2500 mAh NiMH Battery - AA @ $2.95 each = $23.60
https://www.sparkfun.com/products/335
1x NiMH Battery Charger @ $7.95
https://www.sparkfun.com/products/10052
2x Battery Holder - 4xAA Square @ $1.95 = $3.90
https://www.sparkfun.com/products/552
1x N-Channel MOSFET 60V 30A @ $0.95
https://www.sparkfun.com/products/10213
1x Arduino off of eBay from China or something @$13
1x Hoody @ $25
I generally prefer buying stuff on eBay or Digikey because the prices are much cheaper than SparkFun but they had that wearable keypad and I wanted shipping in a reasonable amount of time so I went for it. Also could have separated the ATMega from the Arduino and used it as a standalone but didn't feel like spending the time to do that to save $10.
Basically the way the jacket works is that there are two pads in front and two pads in back. The wearable keypad controls the temperature through the PWM output and the MOSFET. The MOSFET is for switching the control voltage/current to the heat pads. This is needed because the Arduino I/O has a 40mA max current and we need alot more, the Heat pads draw ~700mA at 5v.
Schematic:
Arduino code:
http://pastebin.com/hDat8LC0
