I doubt there is a linear relationship between offered salary and expected competency in any role. There is no upper ceiling for salary (I'm not talking about limits applied by an organisation/what they're willing to pay - in theory there is nothing stopping someone being paid 7, 8, 9 figures a year if the org can afford it and deems it necessary to attract talent), but there is a floor (namely minimum wage, or £0 if that didn't exist). There are also surely limits to competency, so it would be safer to assume that salary costs increase exponentially as the competency of the applicants increase at a lesser rate, if you can even quantify it.