How to say NO to your child?

haiz, there are certain things made in this world, not all is for everybody.  For a child, is really hard to understand the costs of money, really very hard. Its even harder if you have material comforts at home like car, motorbikes and keep telling your child can't buy this, can't buy that!!! The trick is, its not about the money, its whether it is of value to him or not?

1. Having a toy, what is his level of happiness and comfort? Any other toy to compensate, or a movie, or a DVD can? 
2. Does having that toy cause suffering to others? Eg you, spending $80 could mean walking to school instead of taking bus, because the money could have been used for something else.
3.  Have to learn to give up some things in life, you cannot always have what you want, the world does not work this way.   Fact of life, like right and wrong.  Very important.