It's cheaper to buy a pre-built system. :-) However there are 4 thermocouple inputs and numerous additions that are a piece of cake for an Arduino based system. (Bluetooth Remote control interface with serial logging comes to mind). The main designers of this card are building it for control of coffee roasters (another hobby of mine). Coffee roasting is a fairly complex job and the beans eventually generate heat in an exothermic reaction...so I figured I'd control my smoker first. (Btw: This would also make for an excellent Sous Vide controller).
Here is a guestimate of the parts:
$30 Arduino Uno
$16 TC4 board with surface mounts pre-installed, $6 for a bare board, $45 assembled
$15 LCD Adapter board with surface mounts pre-installed, $6 for a bare board
$13 2x16 LCD 5v
$15 Crydom D1210 (new from eBay)
$20 misc electrical components if you didn't get a pre-assembled board
$20 Misc: box, on/off switch, thermocouple mounts, terminal block, wire, crimp connectors, PC power cord cut in 1/2
$4 5v-9v DC voltage wall transformer (old cell phone charger from the junk bin, mine was sitting in a box)
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$133
TC4 Shield code
http://code.google.com/p/tc4-shield/If you are interested in the TC4 and LCD control board see:
http://www.mlgp-llc.com/arduino/public/arduino-pcb.htmlMark