Just have the pro game bind player salaries to 35-55% of a team's previous year revenues. Newly promoted teams can have the median figure for two years or something. Let salaries grow organically not based off what sugar daddy's. Would also give team's an extra advantage to building a revenue base.
Would need far more complicated maths than that, and to sacrifice the (current) purpose of the salary cap.
A) What counts as "revenues"? IIRC Leicester own a car park, who's revenues go towards funding the club, Exeter used to own a hotel, several grounds include conference facilities (which presumably earn way less post-covid), whilst others don't even own their stadium or training pitches.
B) The (current) purpose of the salary cap is to keep a competitive league, rather than splitting off between the "Have"s and the "Have not"s - and TBH, it's doing a pretty good job. You can't just go out and buy league success in the way that, eg. Newcastle did before there was a salary cap (or some would say that Saracens did when they ignored the salary cap).
I'd further add, that... what do rugby clubs spend all their money on?
Even Sale and Newcastle have turnovers in the region of £12.25 Million p.a. whilst Harlequins are double that, and another 5 clubs hover around the £20M mark.
Yes, I know there's far more to it that Player salary + coaches' salary, but it seems that if you've a turnover of 4x the salary cap, if you're making a loss, it's because you choose to.
If Gloucester, say, can have a turnover of £17M for a total loss of £0.6M, and an actual EBITDA
profit of £0.6M; what are Quins spending spending their extra £10M p.a. on to end up with a £4M loss (though only a £0.2M EBITDA loss). Okay, so Gloucester are cutting their coat according to their cloth, and not spending up to the salary cap - but I'm pretty confident they're not £14M below the cap!