NRL cap auditor does that anyway. They put a realistic market value based on a number of inputs (e.g. standing in the game, achievements, reps, PM's etc) on a player before they accept a contract which is not necessarily the highest bidNo to Sam Walker - too expensive.
The Salary Cap should be changed to work as follows - if Sam becomes uncontracted and another team is prepared to offer Sam $1.5M (via a registered bid through the NRL) then should he eventually stay with the Roosters - they should be forced to include a minimum of $1.275M (85% of 1.5M) in their salary cap. If the Roosters pay him more than $1.275M then the Cap amount increases accordingly.
If a team is stupid enough to pay a player 50% above his worth (e.g. Hunt back in 2017 when signed) and he does not go there (like him or not he came) that is there stupid problem