Reading this thread about the cost of parking at Manchester airport being on the rise I decided to book now for early next year. I didn't need to look far to find APH park and ride was £42 for 15 days. That doesn't seem to have gone up much at all. In June we paid £52 for 10 days using Jetparks, and this autumn we have 30 days at Hunters, a first for us, at £71.
With a round trip of 180 miles using a maximum of 4 gallons of petrol at say £20, it's a lot, lot cheaper than any alternatives.
My advice though is book well ahead and shop around.