Well if you have it already, why not use it then. However, keep in mind that you can get a new arduino for around 14$ shipped. I have bought lots of stuff in the past that ended up being really cheap to replace because of falling cost.
Anyway, you are looking at 2-10$ per thermocouple, depending on type, etc, and 3.50 (maybe 7$ i forget) for a max5565 (iirc) chip plus a multiplexer chip and boards, watertight enclosure (10$ at home depot)
That said Im not sure why you would want more than one or two thermocouples (not in meat) in the smoker. Yeah you will have hot spots, etc, but id assume thats constant over time, no need for realtime monitoring, you cant control for it. Once you know where they are, you can account for them.
My 2c.