The NRL salary cap is of course much higher, and some players just don't want to go to England. But the marquee rules mean that wage gap doesn't matter much for one or two players if you have the money. Half a million dollars - way more than a backup halfback would get - is 285k at current exchange rates. Throw in a longer deal to lower the annual salary and you'd have takers at the upper age range.
Of course some players don't want to go to SL. But I don't think that's the issue if we really want to get someone. The issue is how much we're willing to pay.