During my career I have learned that every music requires different level of the focus. It is very difficult to quantify the amount of work to be spend when we do the art. Punk music setup is way too easier than synth-pop arrangements - with full respect to both gengres.
So prior to any contract we need to discuss the range of work. Below price levels are just informative :
Editing / Noise restoration : From 19 USD / hour We accept also
Mixing : From 200 USD / song
Mastering : From 40 USD / song
In case of the mixing and mastering client always has two rounds of the comments/revision included in the price.