a somewhat overkill thermocouple: but for less than $30. . .Also by Omega.
BLMI-INC-K-116U-12 (you'll probably want to extend the leads a bit. . . .)
Omega makes one they advertize for $9, but it might not do well in the dirty environment you're talking about. Besides, it's a little short for your purposes.
Any number of people will sell you thermocouple wire, and you can then make your own thermocouple in whatever shape and length suits you. . . twist the ends together and crimp them in a butt splice or hit them with a oxy-fuel torch or tig. It really is that easy. Type K wire is cheap. (something less than $1 a foot(?) last time I bought some)
Total cost, $150 or so, not including incidentals and overnight shipping. . . and thats NIB.
So, yeah, with a project box, wire nuts, and a cheap extension cord, $200. But you probably have a cord already, wire nuts are cheap, and Ammo Cans and old tackle boxes actually make pretty good enclosures for this sort of thing. . . Flea-Bay and various other surplus sources can probably knock half of the remainder off.