Ok...there's two parts to your question here...
First of all, there's the actual editing and mixing of the project. You don't say where you are, but you are competing against the local post-studios and so their hourly rate should be a guide for you. Do everyone a favour and don't undercut them, because that ultimately harms the entire industry.
Secondly...the music you are composing... Let's be honest here - the client could conceivably use production (library) music and still get the result they were seeking. If they did go down that route, they would have to pay a fee based on the duration of music used, and the number of copies made. This will be administered by the national mechanical copyright owners society - in Australia, this is AMCOS, in Canada it's SOCAN I believe, and I think it's ASCAP in the USA...but don't quote me.
When you have figured out what it would cost to do it that way, that will give you a guide as to how much to charge your client...because if your client HAS a clue, they will know all of the numbers you are competing against already.
So...you have to walk the fine line between charging too much and having the client "go legit", and charging too little and having the local industry wanting to kill you. Sure - they are going to hate you, but at last you haven't got the client used to paying a stupidly small rate and creating that expectation for EVERY job they ever do - whether with you or in a legit studio.
Moral of the story: don't shit where everyone eats.
Cheers,
Tim