I was a notary in AZ for 30 years, then I let my commission expire a few years ago.
That $300 - $500 sounds very high. As I recall, you have to submit your paperwork to the state and enclose your fee (less than $100 if I remember), then purchase a personal bond from an insurance company (maybe $5,000 limit, so actual cost was less than $100), and then buy your notary stamp and record book. That's it - shouldn't be more than $250 all total unless things have drastically increased. Good news is that the training book is free online from the Secretary of State's office.
You're correct that state statute dictates the fees you can charge per signature or per document, but you are allowed to charge reasonable travel costs and for your time if you become a mobile notary. Usually when people need notarizations it can be a time issue, so you'll get requests for same-day work that you may not be able to fulfill if you have a regular job. Even with that, you're putting miles on your vehicle and hours into your weekend that may end up bringing not much more money than a pizza delivery job.
For me it was something I did mainly at my workplace because it was needed sometimes for my job, and as a convenience for my co-workers. I never charged a fee but my employer paid all of my renewal costs every 4 years.
One other thing to keep in mind is that Electronic notarization is on the horizon. It's been happening for a few years now and will be more commonplace after Covid-19. If I were you I would look into that aspect.
