Mastering takes a mix you think sounds great until you play it next to something else and wonder why it sounds so boring and makes it great.
It takes a sympathetic but preferably external ear and occasionally the willingness to challenge the artist's preconceptions. Part of my job is often to query the artist if I hear things which might be improved, or which at least might be worth thinking about.

I learnt to master when the person we were paying to master the 3rd Glow album (Enteiticada) did a lousy job - all the necessary clarity but none of the warmth (and wildly varying apparent levels).
It was something I took to fairly readily and after mastering my next 2 albums and a host of old projects I did my first professional mastering job in 2005 for Jazzie B (SoulIISoul).

Since then I have mastered a wide variety in style from live, world dance and electronic psy-chill albums (Alejandro Toledo and Terra Nine) to a lot of South London Hip-Hop
I have also mastered everything produced here even when released on other labels including Orchid-Stars Birth and all remixes.

Southport Weekender Vol 3 Terra Nine - Breathe - Altar Records Orchid-Star - Birth - Liquid Sound Sirius Music - Lotus

I am happy to work on high-end professional releases or projects not demanding such perfection.
I charge by the hour but work to a client's budget.
I will advise if I don't think the budget doesn't fit the requirements; I'll tell you if I think it needs more and I have on occasions brought a job in under the agreed budget.

Cost

Take £25 per track as a starting point.
That will vary depending on the quality of the mix and various other things including how important this is to you!
Usually the more tracks being mastered the quicker it takes. However if the tracks vary widely in style or quality it may end up taking longer if the tracks need to sit together comfortably.
I am also happy to advise a client in advance of ways they can best make the job easier for me (and cheaper).