Mastering is preparing the master tracks for different media such as CD, vinyl, radio, etc. No one can actually master other than a mastering engineer in a mastering house. Mastering is just about the most incorrectly used term in production.
And you can get Cubase for $300 at academicsuperstore.com
They also have Cubase Studio for $200 and Cubase Essential for $75. I don't know the exact differences off the top of my head, but I'm pretty sure the Essential version would be more than enough for most people.