While the term "mercenary" applies to a large number of people and units, with varying levels of skill, professionalism, dedication either to their employer's interest or religious profession, there are several sketches that are illuminating.

Here's a fantastic collation of information, with proper sources. It talks about the ideal food supply, the salary, and unit composition.

R. Bonney's "The Thirty Years War 1618-1648" provides sketches of several famous mercenaries. You can read an excerpt here.

The ability of states to fund their mercenaries reliably changed drastically over time, to the point where mercenary commanders had to choose their routes carefully so that they could forage and raid countrysides that are less ravaged by the war.

Experienced mercenary units are highly sought by both sides.