It depends on a number of things, such as geographical location, unix distribution you know, etc.
A lot of the ads you see are frequently written by hiring managers/recruiters, so it isn't necessarily that an ad accurately describes a job.
Most of the people I know in the field started as either support desk or NOC - I started in a NOC myself . Usually, the knowledge level of the support desk or NOC doesn't go beyond that of a regular user.

Ex: In Solaris, if you know this part well, you can do support desk or NOC:
http://docs.sun.com/app/docs/doc/806-7612

Knowing this part well will make you an entry-level admin - even without knowing the GUI crap - :
http://docs.sun.com/app/docs/doc/817-1985

As for the money, it depends on what you do, when you do it, and who hired you to do it. There are shift differentials, bonuses for or paid on-call, etc... security oriented work usually pays higher... be prepared to do app support as well.