And for the record, I wasn't kidding about how sophisticated this BBQ stuff has got. Below is a pic of my wireless setup. Two probes: one for meat and one for BBQ ambient temp (or one for each of two different types of meat). You can set alarms for each. You can set timers. You can use USDA meat temp alarms. It alerts you if the probe disconnects. It's backlit for night-time viewing ( a brisket can take 16-24 hrs.).
And mine is low end; some have many more probes, integrate with the smoker/BBQ controller, have Bluetooth, monitor humidity, and at least one on the market has a USB port where you can download the data to a thumb drive so you can graph the perfect brisket for YouTube bragging rights. I'm dead serious here—
graph your BBQ event. Interestingly enough there is a lot of science here as meat goes through plateau or "stall" phases that makes sense if you see the graphs. You can work in the shop with the receiver attached to your belt as you weld or machine away...always knowing your ribs will be perfect. (Also useful if you are having too many beers with friends as the BBQ toils away in the background.)
A custom smoker can easily run over $10K. If you understand the electronics of the controllers, know the basics of good BBQ (as opposed to just grilling), and are handy at metalwork, they are terrific projects and potential profit centers.
I don't care what anyone says, BBQ on your own smoker is worth every hour of labour you put into building it. This ain't your Daddy's grill anymore.