Cory,
I don't have any very cold winter experience with diesel so I can't give a real good answer. Personally I would Run it down to about a quarter and put that anti-gel additive in. Then when you go get it you can top off with fresh diesel.
As to the cost. I feel your pain. I am in Virginia. I keep mine as full as I can. Maybe a little less now that it is getting hot, to allow for expansion. It took a while to get it full. I put some in each payday until it was full. Now I don't have to add more than 20-30 gallons at a time. About once a month.
See ya,
Chuck