Mehdi:
You have basically two methods to use that are fair and legal.

One of these is referred to as T&M (time and materials). You charge a fair fee per hour for your time and expect 1 for 1 repay for cost of materials that you expend to do the work. This type of work/pay method works best for projects where the definition of the work to be done is vague or uncertain.

The other system is called a FP (fixed price) project. You decide up front how much you believe the result of your work is worth to the customer. Then based upon this value realized for the customer you negotiate a price and terms for the service to be performed. The customer acknowledges the value of the proposed work by agreeing to pay the price you set.

Clearly there will be differences of perceived value for work projects and/or work hours based on factors such as your expertise, your past portfolio, geographic location, and financial means of the customer. These all have to be taken into account for each individual situation.

Michael Karas