A 1:250 dilution rate for IPA is 0.4%, which will have hardly any effect on the freezing point of water.
To get the water freezing point down to say -4°C it would take a 10% dilution which would mean 40litres of IPA in a 400litre tank!
If you were buying 40 litres from WCW it would cost you £172.32 (inc VAT).
So, it's cheaper to stay at home and wait for the weather to warm up when it's sub zero