Shalwan: let me give you a quick lesson in the fundamentals of negotiating in business. Never sell yourself below the industry standard or your client will take advantage of the situation. Use the formula I have given [or tweak it] and bring that to your client when you are billing them. They will think you know what you are doing and you aren't some kid/student who is only trying to make a few bucks. Oh and one tip especially if you have a "big" client don't forget to brand yourself with your company. Good luck mate,
Just add one more multiple to whatever formula you use - your eagerness to get the job. Sometimes freelancers do their first project(s) for a minimal fee, just to start building a track record.