I just read your response to the iPad post, and I wanted to share what we've done with DS to help him understand money. I'm not sure if it will help you or not, but if it does, then great...if not, then forget I ever posted this.
I feel like DS has a pretty good grasp on the idea of things being expensive once you point out how much something costs, and to get here, I've (mostly) used the idea of cheeseburgers and ice cream cones, which are huge treats at our house.
We've also worked with him to understand that we only have so much money, and once it's gone, we can't buy anything else including food, gas to go places, etc. It's helped him understand that we have to be careful to not spend too much on "treats" because then we wouldn't be able to take care of things we really need.
Ex. We are at the store and DS asks for a toy that costs $5. I explain that for the same amount of money he could either have that toy or 5 cheeseburgers or 5 ice cream cones ($1 each). Usually he responds in a shocked tone about how expensive that toy is, and then I ask what he'd rather be able to do...Be able to go get ice cream 5 times or get the toy, and almost 100% of the time, he'll choose the ice cream, which I usually do allow him to do once in the next few days anyway.
Now, with the iPad situation, my explanation fails b/c he has no concept of 500 anything yet, so I tried to compare it to all the food that we buy at the grocery store for a whole month, which at least left him in awe even if he didn't "get" the numerical value.