Thanks for the tag (I didn’t see this posted until I got an email). Marko is spot on with his comments. Allow me to go into a bit of more detail.
The short answer to your question of whether or not there are independent contractors as FSO’s is no because the NISPOM doesn’t allow it. Now that being said, the much longer answer is that you can be an independent contractor and consult to a company’s FSO that is in place. This happens a lot, as contractors are required to have an FSO but often are too small to hire one outright and/or just use a member of the management team since they are designated as KMP’s. NISPOM requires that the FSO be a part of the FCL and an employee. This may be achieved via part-time work but remember, you can be walking on thin ice if you want to work for multiple companies this way (as the DSS will know if you are listed as multiple FSO’s).
You’ll see companies advertise as a “FSO consultant” and typically what happens is they are hired by a small company to guide the registered FSO. Especially for small companies starting out, there is a lot of things to be put into place. Additionally, there are the self-assessments, etc.
I cannot speak for everyone, but my career started by me essentially falling into the position because of my years of experience in management, law enforcement and criminal justice. No one wanted to be the FSO because there is so much involved and so many responsibilities. It really isn’t a bad gig, but many just don’t want to bother with it. Some get their start by being appointed as an Alternate FSO, get all the training, then branch off. Salary wise, I have saw it range widely depending on the type of company, area, etc. A company that hires a full time FSO to just do that job can range anywhere from $50k to $110k+.
I know that doesn’t really help pin down your question on salary, but it’s really dependent on a lot of things. For example, I am an FSO for multiple entities within my organization (mid-size corp), thus my salary in a higher range. Some companies require FSO’s to have cyber/IT certifications (many companies are going this route). Some may just do mostly administrative duties in an office setting, where some may be out in the field.
To get started you’ll have to find a company willing to appoint you so you can get your training (gotta do the FSO training), and you’ll need to get familiar with the NISPOM.