The first and most obvious job that pops up in the Linux field is a Linux System Admin. A System Admin can be a lucrative career, especially if you can couple some security knowledge/ certifications with it. Positions are advertising pay ranges of $80K per year with about 5 years of experience and $40k at the entry level.
Another category of Linux job is a Software Engineering. These jobs are paying around $80K per year as well, with a few years of experience. They require some type of programming knowledge in C+, Python and other languages to be coupled with Linux knowledge.
The highest paying job I found was a Linux Drive Architect, although this job required a masters or PhD in computer science. The pay range for that particular job was $120K and up and it was located in the bay area.
There are a wide variety of Linux jobs available, especially in the web based world. Many web servers are running Linux as it helps protect the website from becoming infected with the run of the mill Windows viruses that circulate the web constantly. Many web languages are also designed to run on Linux servers, so a person with some programing experience coupled with Linux is a valuable commodity.
Linux jobs are scattered all over the country, though the highest density of jobs is in the California Bay Area.
