It depends on how good you are at mixing and the amount of time you put into it. And I would charge it as once price because you aren't actually mastering if you're the one who mixed it.
Do it free first! Grow your clientele and get better at what you do and get experience working with clients first. Then start charging a little, 50-100$ / song for example. That's what I did
Do it free first! Grow your clientele and get better at what you do and get experience working with clients first. Then start charging a little, 50-100$ / song for example. That's what I did
A fixed rate is good, if you can work really quickly within your DAW! The other guys have offered good advice, but if you have a skill your time should always be worth something.