hmm, thats kind of a tough call, i mean different songs will be harder to mix than others. i mean if your producing the beats then most likely they wont be able to mix it themselves. i would say th best thing to do would to think about your time invested. maybe you should do it by an hourly wage. i mean you dont wanna charge somebody something ridiculous, but i know from experience that mixing and mastering a track is one of the tougest things to do, and can take plenty of hours....
either way, good luck...