I have a Raypac heatpump. I have not used it during a cold season yet but the salesman and my friend (who has one) both said it runs about $1 a day in the spring and fall, and $2 a day in the winter to maintain 80 degrees. I figured that in those cooler months I won't be using the AC so the cost of the heatpump will just about offset the savings. I did use the heatpump in April when my new pool opened but the water temp was already hitting 80 degrees in the afternoon so it didn't get much use.
Good luck.