I don't think salary will be his motivating factor - it'll be competitive aspects.
Lawwell has shown zero support or desire to any of his recent times managers the second they want to take the club beyond being whipping boys in Europe. Even Rodgers the Messiah got told to do one.
For all his wealth, DD doesn't just pump in huge sums of Sugar Daddy money to the club. He insists they're run at a sensible financial level, so the notion of him just stumping up tens of millions to give Benitez the kind of kitty he'd need to do anything of note at CL level is a non-starter.
He can continue to work with more budget in the most lucrative domestic league on the planet, take Newcastle into Europe and keep building them, no doubt looking at the likes of Spurs as inspiration, or he can be getting 7-0 pumpings off teams in Europe on a Wednesday and then dragging his team off to Hamilton to get kicked about a plastic pitch in the pissing rain on the Sunday.
There's a reason BR was happy to jump ship to a mid-table EPL side in the middle of the season without a moment's hesitation and it's illustrative of why a guy like Benitez (who want see salary as a primary motivator) won't come near.