The answer is simple. You need to charge to total amount your customer is willing to pay for!
Before saying the price you need to understand the country your customer is from, the size of the company and the project. You need also to compare your level of expertise among other developers.
If you are fast and deliver high quality coding I can guarantee you will have more customers willing to pay the price you set than the total time available for coding.
I suggest you to start with low rates, get some customers, show your work quality and start to increase the price.