Ok - first off, I doubt you are going to be able to find a sensor that will do what you want while still being inexpensive. If one does exist, it probably works something like the oxygen sensor in an automobile - in other words is it probably an active sensor of some sort. Having a pellet smoker myself, I just can't see how you would be able to stick a sensor inside such a smoke-filled chamber and have it work for very long (as the condensate from the smoke would likely "gum it up" after a short while).
That said - check out this article (if you haven't already):
The key statement is at the end:
Since it is hard to measure humidity directly or inexpensively under cooking conditions, measure the water loss rate, and use that as a surrogate humidity level.
This of course means using a pan (or something) of water, then somehow measuring the water loss. Even then, you wouldn't have a perfect measurement, because that would say nothing about what moisture the meat itself is giving off.
Perhaps you could do a wet-bulb/dry-bulb measurement with a couple of thermistors?
High-temp thermistor probes are fairly easy to purchase; alternatively, you can use a thermocouple and amplifier. You'd have to figure out a way to keep the wet-bulb side wet, but that might be much easier (and cheaper) than a real high-temp humidity sensor.
...is using this sensor:
It's not really clear where he is measuring the humidity, given the max operating temp of the sensor (I'd have to guess at the stack, where the temp might be lower?)
Ultimately, my gut feeling (nothing objective here) is that the more you watch, the longer it takes to boil. In other words, maybe going to all of this trouble of checking, measuring, etc - you're ultimately doing yourself a disfavor? I understand that there is science behind cooking (and of course smoking), but I think pursuing the perfection, at least for myself, kinda takes the fun out of it.
What I've found works best - at least so far, with my trial-and-error methods:
Low and slow - I put my smoker on the lowest non-smoke setting (180 degrees), and let it run for 10-12 hours.
If I think I need or want extra moisture, I'll put a pan of water in with the meat.
Or, I might put a pan of "sauce" in with it, and baste on occasion until the sauce runs out, then move the meat to the pan to keep the humidity level up.
Really, though, my best results have been just to set it to 180, throw the meat on, close the lid, then walk away and don't peek at it until 10-12 hours have passed. But hey, if you manage to come up with a great automated monitoring system or solution, I'd love to hear about it. So keep experimenting; I'm not trying to stop ya!
Lastly - I'm going to leave a few more links; maybe you've seen them before, but if not - enjoy: