Sensible is obviously way behind the times, avgas prices have gone up - by an average of 5 - 8 p a litre - in the last two weeks, this is entirely due to the spot price of oil on the futures market going up by a similar percentage.
Do you think that the suppliers (airfields) are benefiting from this? The short answer is no, in fact their profit margins go down considerably in such situations and often they lose out entirely.
Pay the market rate, just as you would for your car, (for Red 4 and Sensible's benefit, supermarkets dont supply avgas), or try and arrange for your own supplies, perhaps then you'll realise exactly what's involved.
P.S.
Red Four speaks out of his/her hat, free landing fees maybe, but any pilot of any worth would sort out safety first and worry about peripheral factors second, to do anything else would be grossly negligent.