The definition of what something is worth is what the market will bear.
Most people do not define worth based on monetary cost; you are using a very narrow economic definition. If you can't get past that word, then please substitute "value", or "usefulness", or some other equivalent above.
Now, it should be noted that this only works when there are a multitude of buyers and sellers, but that problem is a different problem and should be recognized as such, instead of suggesting employers pay more than market rate, maybe we should be focusing our efforts on the real problem of monopsonies. Or if people are inherently "worth" something just for being, then I think it's more accurate to refer to that as "What are people entitled to" rather than "what are people worth" to avoid muddying up definitions.
If you ask someone if it was worth it to have a plumber fix a pipe, and they say 'no', what they're telling you is whether or not their time is worth more than the price of paying a plumber. If it's possible for someone to believe they are getting more value out of a transaction then they are paying into it, than how can value be defined by what they pay?
If market prices actually determined something's value, then you would never ask if hiring a plumber was 'worth it', because by definition the answer would be yes. After all, by definition the plumber is worth what you pay them.
But you do ask, because most people recognize that when they define 'value', they are referring to something external which the market can provide them. Winning or losing in a transaction only works if you believe the value of what you get can be different than what you pay.
I dislike tautological definitions because they're not useful and they don't provide any new information. The claim that OP was trying to make was "what most people think of as value (for example, 'how much money an employee makes a company') does not correlate to pay." That's an interesting claim because by default people who are operating under principles of fairness might assume otherwise.
To say 'everyone is payed what they're worth by definition because what they're worth is what we pay them' supplies no information to anyone and is not an interesting claim to make. It's technically self-consistent, but brings nothing new to the discussion.