Where would the money come from? Good question.
It could come from two sources: external investment or increased revenue generation.
RL won't attract the super-rich to invest in it or attract a significantly higher level of revenue unless there is a step-change in its management and administration.
We need a better international game.
We need clubs to be incentivised to raise income through a reformed salary cap - one which actually prevents clubs from going bust by forcing them to comply with business plans that have been approved in advance by independent auditors. If people want a level playing field (though we don't have that now, small clubs are still not getting through to grand finals - well, unless you count Saints

) then the cap could be raised slowly.
Long term though something must be done. Forget RU as a threat to RL: if the cap isn't raised in line with inflation then before long the game will become semi-pro anyway. The fact is that the cap is now worth over 30% less in real terms than it was when the cap was brought in. If this continues over the longer term then many more players will need to get jobs or will simply drop out of the game.
What a damning indictment it is of the people running the game that clubs can now afford to pay a third less than they could when the cap came in. Anyone that thinks the SL era has been a success is wrong - that cold, hard mathematical and economic fact is condemnation of over a decade of mismanagement and a devastating lack of ambition.