how do i decide what to charge a client???

now i know this is a highly controversial topic but here is the back story....i have been recently approached to shoot someones baby shower event that they are having in march at a local establishment. they are looking to retain me for about 3 hrs. im an intermediate amateur if that makes sense lol, i dont want to turn down the client because i need the experience and money for that matter. its not a ritzy event. but i do not know what to charge this lady. whats the best way of developing a way to price my services. i know alot of variables come into play, how many guests, how many photos do they want, do they want a cd or album, is travel involved etc...is there a template to follow per say. or a sound way of doing this?