Quite a bit, as Atlantis will tell you. Hundreds for a few songs, more for a whole LP.
In my mind it’s only worth investing in a 3rd party if you’re releasing an LP, otherwise you’re better off learning how to mix your individual tracks better all by yourself. It will improve your song writing too.
I use Audition 1.5, with vst and dx support. With the 32 float you can get really smooth sounds. There are pleanty of good vsts out there to give you a nice analogue sound if you want it, Isotope Ozone 3 is a worthy investment if you want all kinds of grunt and dirt. For real dirt, get real analogue hardware - nothing beats it. But, as a fidelity freak I’d suggest that everyone concentrait on digital-clarity.
Having good reference monitors is the key, in a workable accoustic space (i.e. no harmonic reflections). Focus on EQ first: A-B reference your tracks with “pro” tracks of the same genre. Then, hunt out as much unnecessary bass frequencies as possible. You will then get more lattitute for hard-limmiting-boosting with the overall db.
Use EQ to then hunt out harsh high-mids. Eg. if you are using distortion, you can get nastiness around 5k. Sometimes small notches around 300hz and 600hz can be tasteful, as well as 3k. Be patient, use subtle notches on a parametric equaliser, and hone into what each track needs for ultimate listenability.
The final step with EQ is boosting the trebbles, but gingerly. Anything above 22k can be boosted to give that crisp modern sound. At this stage, if you are finding things are too harsh you’re probably using cheep synths and samples (which can be cleaned up a bit in the premix).
Then, take the same gentle experiementing approach to compression and limmiting (which I will not write an essay about here). Presets nearly always get you into trouble - learn to control and understand each parameter.
Check your final mix against pro tracks, and then in a few days time when your ears are fresh.
Get help, ask other music enthusiasts how your mix sounds. They will point out things you are overlooking.
Be careful with mastering studios, they may dirty up your sound in undesirable ways. There is a great deal of work out there that is mastered very poorly, and very loudly. This does not equate to good.