I am not sure if this will apply nationally but certainly all the Tesco filling stations in our area now charge 20p to inflate your tyres and the same machine charges £1 to dispense upto 5L of screen wash. The majority of cars washer bottles wont hold 5L. Therefore if you take an empty container with you when you fill up you can dispense the full 5L (Fill your washer bottle first and the remainder into the empty container) This can be stored in you boot and used the next time you run out of screen wash and your windscreen is covered in salt spray!!
I dont think you will find 5l of screen wash cheaper than a pound anywhere.