Tautologies tell us nothing. Why is a given job worth more than another?
You are buying a specific product.
Do you buy the product from Supplier A who charges £10, or Supplier B who charges £5?
If you said A, you're either a liar or an idiot and there's no point in continuing the discussion.
- Though if you said "It depends why A costs more", then that is a very good question.
According to economic theory, jobs are the same. When hiring somebody to do a thing, you want to spend the least overall.
That might mean hiring one person instead of two, paying the more skilled person more than you would have laid either of the less skilled ones - because a 1.5 multiplier is smaller than 2.
You might pay a very skilled person a lot because the cost (in time and money) of replacing them is very high.
The goal remains as spending the least.
However, none of the actors have perfect knowledge and they all have biases (inertia, not wanting to do certain things, wanting to live in particular places). In general, this tends to lead to employers offering more, and potential employees and contractors asking for less.