Mastering Recommendations
1.) Infrasonic Sound
I've worked with Dave Gardner & Piper Payne here. They are great and able to work with anyone. Their rate is the highest, starting at $120/song, depending on the project.
2.) Ian Vargo
Ian's a great talent and has worked on all of Chris D'Agostino's masters, he's middle of the road in terms of cost, around $75/song.
3.) Online Algorithm
These are the cheapest option (usually about $20/song) and quality is inconsistent. Here's a non-comprehensive list of the most popular ones...