Are faculty salaries negotiable?
This entire answer is US-centric, since I have no clue how other countries work. This is not to say that I know the US system that well - but rather that it's the only system I know at all.

Public universities work differently from private universities. Public universities usually have pay scales and these are often public since professors are considered state employees (yes, my salary is public knowledge). So you can start digging around there. Usually there's a range within which you can hope to negotiate: moving out of that range is beyond even the power of the university to authorize. 

Private universities usually have more flexibility and it might be harder to get the range of salaries. But if you get an offer you can ask around to get a sense of the ballpark. 

As for what a reasonable range is, getting data is very discipline-specific. For CS, the CRA puts out stats fairly regularly and there's the Taulbee survey. For other disciplines, there might be something similar. 

Salaries in academia at least at tenure-track level are less flexible in general. Probably the best way to get to the top of the permissible range is to have another offer. Other than that it's hard. Often, the things that are most easily negotiated are other parts of the startup package. Which is not to say that you shouldn't negotiate salary - but unlike in industry, it's less likely to be as beneficial.