I like the salary cap, and fully approve of it - I think it should be connected in some way to the rugby income of a club (RFU, tickets, TV, sponsorship etc; but NOT conferencing, parking etc), and I like the French method of owners putting up the year's outgoings in advance, and protecting them, so that no club can go bust during a season. I also like the French method of owners putting money into a club via donations NOT loans.
I also think there should be leeway for clubs to exploit, but in a money-sharing type scheme.
I'd do something like (NB, this is NOT thought through well, in advance).
Club's salary cap for a season = £20 per ticket sold in the previous season (Prem & EPRC) + 2/3 that club's share of central (RFU, Gallagher & TV) income for the year.
You can then spend above that, but for every £1 you spend, you put another £3 into a central fund, to be split amongst your rivals, up to a maximum of the value of your principle sponsorship deal.
If you attract bigger crowds, you have a larger salary cap (though you still have to fund it). If you play more knock-out matches, you have a larger salary cap (though you still have to fund it).
If your sugar daddy wants more expensive toys; then they can put their name on the shirt, and call it a sponsorship rather than a debt, and spend more - whilst also paying for their rivals to improve their squads.
So, back of a fag packet maths.
Current TV deal is £200M for 5 years (CVC takes 25%?); so £1.5M per season per club => £1M of salary cap.
RFU payments to PRL is £33M per season (CVC takes 25%?); so £2.475M per season per club => £1.65M of salary cap.
Sell 10,000 tickets for 9+4 matches a year, but fail to reach any finals => £2.6M for the next season (regardless of how much those tickets cost).
Sell 10,000 tickets for 9+4+2 matches a year, and a home match in both Prem and Europe => £3M for the next season (regardless of how much those tickets cost).
From that point, if your main sponsorship deal is £2M pa (such as Allianz with Saracens 7 years ago); then you CAN spend an additional £2M on salaries BUT, to do so, you've have to gift an additional £6M to your rivals.
IF Red Bull / Dyson / Lansdowne / Wray wanted to spend £10M on player salaries - then they could. They'd have their £3.65M from central funding, and £2.6-3M from being popular; but to find that additional 3.35M, they'd have to give that money to the club (NOT loan) in the form of sponsorship AND gift £1.1M to each of the other 9 clubs.
ETA: There's also the Gallagher naming rights - but I've no idea how much that's worth - if significant, then drop the amounts from the other sources to still get the similar ball-park figures.