Dave goes to the dentist and says to the dentist "How much to have a tooth extracted?"
And the dentist replies "Well, an extraction can be quite painful, so you need a good anaesthetic, and I need to use special dental tools, so usually it's £100 for an extraction"
"I'm not paying that much!" says Dave, "How much for only half the amount of anaesthetic?"
"Well" replies the dentist, "I wouldn't recommend it as half the anaesthetic wouldn't really have much effect, it'd be terribly painful, but if you insisted then it'd be £60."
"I'm not paying that much!" says Dave, "How much for NO anaesthetic?"
"Well" replies the dentist, "I wouldn't recommend it, as NO anaesthetic would be intolerably painful! Excruciating even! But if you insisted then it'd be £20."
"I'm not paying that much!" says Dave, "How much for NO anaesthetic, and just using a pair of old pliers to pull the tooth?"
"Well" replies the dentist, "That'd be almost impossible. Excruciatingly painful, and no guarantee that an old pair of pliers would get it out first time, and I might accidentally end up breaking the tooth out bit by bit - it'd be a horrible, gory, long and painful experience! But if you insisted then I'd do it for just a fiver!"
"Fine!" said Dave, "A fiver.....you're on.....book the wife in for Monday morning!"