You need to deicde what your time is worth. It is good to know the length of time it will take to come up with the design. From this you can either arrange a charge per hour with the client or a fixed fee. When I first started I was charging $35/hr but quickly realized (with my client’s input) that the labor was being charged at almost the same rate, so I had to charge more. Today I enjoy the design process far more with a fixed fee and I find the clients are happier with this.