Good luck on negotiating salary. Every position has been given a designated salary range. If you don't fall in that range they will just move on to the next qualified person.
The only folks I know that can negotiate their salary are engineers. Why because they can invent something for the company which brings in endless revenue. Everyone is just there to keep the lights on...including CEOs.
Uhhh...I doubt engineers are the only ones who can negotiate salary. The last start up I was at was paying $100k per C-level position. And that's just the recruitment fee, it doesn't even include retainers and other expenses. If a small start up is willing to pay that much for a CFO or CMO, I'm sure other companies will offer a healthy package to them.
So no, engineers are not the only ones who can negotiate their salary. My ex-coworker joined a tech company this year as an accountant mgr. They gave her $100k, she negotiated $120k. She has a CPA and have the international experience they needed to grow their infrastructure so they need her more than she needed them.
I negotiated my salary when I was a senior analyst. I asked for 15% more than what they were offering and they gave it to me. I knew they need my experience and because they didn't offer better compensations like bonus, I got the increase.
The way I approach salary negotiations is that I look at the market first through whats available online (not always trusty, but gives an idea), if you are using a headhunter you can ask them (again not always 100% trusty but gives an idea). For me since I have a network of professionals in my field/market and with the same level of skill sets, we share salary information among ourselves, so that we have an idea. If we go into a company and they aren't willing to negotiate a fair salary, we just move on. Most of these companies will end up with second rate employees that will be a pain to work with anyways.
Another approach is to also interview with multiple companies so you have a comparison. When i was interviewing a long time ago I had offers from two company for the same position but the other one offered 10% more so I went with them.
But to add some validity to your statement, if all things are equal and there are two candidates, companies will chose the one that fits their needs the most...i.e the cheaper one. However in my experience interviewing candidates typically ONE stands out among the rest.