Commercial.
I understand that commercial revenues can drive increased spending on salaries but the next target after Spurs are the lovable neighbours.Would it not be commercial deals? IE Chang is up at the end of the season, and am sure there will be other commercials which will be getting negotiated to increase revenue.
Apparently the wage bills in 2015-16 were as follows (just taken the figures from a random link so they might not be entirely accurate but give an idea of the scale):
Everton 75m
Spurs 110m
RS 152m
That means within 2 years we are faced with doubling our wages bill just to catch up with them even if they stay at the same level. The other four are another jump above (all around 200m). That is a vast amount of commercial revenue to be talking about.