Salaries are determined by the market, but no one ever mentions how the market determines it... which is usually some combination of scarcity, quality, and, more realistically, how many connections you have, etc. These things give you bargaining power which you can use to negotiate salaries.

(This is all very basic stuff but it's more fleshed out than just saying 'the market determines salary'. The market isn't a magical entity, it is run by human behaviour.)

"The market" sucks in situations where bargaining power is unequal, the participants don't have all the information and transaction costs are very high.

Salary is higher in areas where the source of money creation is. The current source by our economic design is currently in the banking credit area. Then the business type, telling the story that appears to be the most interesting for these kind of guys, gets most credit, with this credit, all saleries are paid (so independent from real market or need, just based on a 'market' of imagined future value, if we would sell it would be..remember finance crises, people lost real jobs because money creation stopped). So it is no wonder that health care workers get less and others a lot more. Its not really earned but shared along story telling lines. One fairy tail is "market". Unless the banking investment guy gets sick, once in a while, he has no interest for biology or health care, but he likes big machines and cars. Its hard to imagine how biology is usefull for him, unless just to make a lot more money to buy more cars and penthouses, why not 'invest' in that directly?. Just my little theory :-D

They are determined by the market - mostly - so how much you are paid is going to a function of how replaceable you are, and how much more money the company makes with your position filled or empty, notwithstanding dirty tricks that are somewhat rare.