We all know people who we feel should be paid more, a lot more who should be paid less.

But how do you establish your value to a current or future employer? The only definitive way I know, is to put yourself on the market and see who bids higher.
To make any comparison,it has to be for the same skill and experience mix, and both employers have to value that mix equally.
A lot of job moves are coupled with an increase in seniority and responsibility. If your current employer didn't want you for an equivalent role and made a counter offer, that wouldn't make them greedy, it would mean they are dumb.