Based on the fact that you are trying to keep the cost down, here is what I recommend for mastering:
Use Sonar and Voxengo Elephant and (free) Voxengo Span.
Sonar has all the necessary vst tools such as: LP EQ, LP Multiband, various compressors, various equalizers.
Here is a simplified example of how to go about using Sonar for mastering:
Mix and render your song to a 32 bit file.
Open a new project in Sonar and name it: "Master_My Song", or whatever you want.
Import your rendered 32 bit mix.
Listen to the song and decide what it needs.
In the main subgroup: Add EQ
if necessary, add "glue" or multiband compression
if necessary, add stereo widening
if necessary, add whatever else
if necessary, finally add Voxengo Elephant.
Automate volume of song in Sonar
if necessary, example: make choruses 2 dB louder than verses.
Bring the input level up in Elephant to hit your target loudness (maybe -13dB RMS average, but that is your call).
Notes: Voxengo has a real-time display of RMS level, You can set the input level and watch the RMS level to get it roughly at the volume you want.
Voxengo span (and your ears) can help you find where EQ may need adjustment, such as low-mid build-up.
Place Span after the EQ but before the compressor and Limiter.
Voxengo can also be set to K-14 if you choose to use that method of setting loudness.
if you can't reach your target loudness without making the mix sound completely squashed then you need to go back to the mix and modify it.
if you are mastering multiple songs you can import them all in the Sonar mastering project and keep them all muted except the one you are working on. The goal with multiple songs is to get them all adjusted, tone-wise and volume-wise, so that they all play well together and that none of the songs stick out like a sore thumb.
I hope this helps. Read everything you can find on mastering, Watch all the videos you can find.
PS. I discovered that we have a Blue Tubes brickwall limiter included with Sonar.
I have not tried it but it may be good enough to use as a mastering limiter.
Give it a try.
Now you have a mastering environment that didn't cost you anything extra.