How cold is too cold to leave cannabis plants outdoors overnight in high desert humidity?
High desert humidity? 
I’m a few hills west of the high desert and we rarely have humidity above 21%.
It gets down to the 40s overnight in Humboldt in October. Just sayin’.
We usually stay around 50 -60 % humidity. Foothills Western Wa.
Dehumidifier is your friend when trying to dry.
I agree with put a cloth over them. Generally a hard frost is 28 degrees or lower for 24hrs. Most plants can tolerate a little cold, not freezing 32 degrees