If you're confronted by guys with knives you're going to run away? It's generally safer to give them what they want and be done with it.
I've used this technique sans the $40 dollars. One thing to keep in the back of your mind, "do they have a mobile phone?". If they do, a football field might not be enough. You have to move out of sight, then change directions ~ http://www.flickr.com/photos/bootload/3497187984/
Although in a real situation you might not want to test anything from this thread, because you could be very unlucky.