Of course it’s not free but it’s barely noticeable. The cost of keeping a fan running once it’s gotten started is minimal and when you factor in the furnace running less cycles per day it becomes a wash or better sometimes. Then there’s the comfort of having even temperatures throughout the house. That in itself is worth the times when it does perhaps cost me a few dollars a month. I recommend you give it a try. To gauge the cost you’d really want to start in a month where there’s no real demand for heating or cooling so you can see the impact on your bills. If you start now (winter) it’s hard because of course your bill is going to be higher than last month so how much of that is a result of running the fan is hard to detect. Perhaps you can compare to last year at this time and see assuming your electric hasn’t gone up.