It depends on what they want. Are they necessities, things for school, basic needs?As parents we have the responsibility to provide those things. It is good to teach children that they can have power over their lives by working to get the things they want (within reason). They learn to be discerning in what they want, and they learn responsibility. They will also take a lot of pride in the item is they earned it.
We would let our kids know that we would help if it was something important. If they wanted a new CD player and showed a lot of initiative in working to earn the money, we would help a little bit so it wasn't an impossible goal.