I have a Rheem 10 Seer Heat pump about 10 years old. It has been in the 20's & 30's the last month and my electric bill was $700. The bill before that was $200 and the temp probably averaged 30-35 that month. I know the auxillery heat will kick in when the outdoor units are not heating enough but why would they kick in every time the temp is only 1 degree difference or when it's the same as my set temp. Also it appears its only happening with my downstairs unit. My home is only 2400 sq. ft. I know the auxillery heat runs up the bill but $700 is quite ridiculous. Also I noticed that the copper line on the one unit is always cold kind of icey and the other unit is warm. Any thoughts?

Not an uncommon event, particularly with a 10 year old unit. Aux heat comes on at 32+/- and burns the KW. There isn't much heat in 20 degree air. The only solution is to turn the heat down and bundle up or replace the older unit with a newer cold weather model.