Some songwriters say they have received amounts equal to a ten-year advance on royalties. The money has been used to take their career in a new direction, buy new equipment or fund a tour or music video. However, not everyone may have a roster of songs that will sell for enough money to pay the electric bill. Keep in mind that once you sell your rights to your intellectual property, you waive your right to collect any future royalties it may generate.
It is generally recommended that you hire different engineers to do your mixing and mastering, or at least use different sound systems. One stage focuses on the combination of individual elements and the other focuses on the sound as a whole. One looks at trees, the other at the forest.