Can a $20 torque wrench from Harbor Freight, etc., be good enough for lug nuts?

Actually they work quite well. We used them in the factory after testing a number of different brands, including some that were quite expensive and their torque remained within tolerance as long or longer than any of the others. They are not as comfortable to use as some other brands though.

In the end, we settled on the brand Utica, comfortable and reliable.