Somewhere (was an older thread on players capping) I reviewed the total skill points and salaries of allstar level potential players on the TL. There was a good sized sample of players with high salaries, and I would guess that most were likely capped.
From comparing the total skill points and the total salaries, it appeared that what controlled capping wasn't salary per se, but total skills. Guys who were focused only on primaries were able to get to very high salaries, but players who were more well rounded ended up at lower salaries.
Now, there are plenty of unknowns in this, such as potential sublevel, skills sublevels, and whether they were truly capped or not. But it was enough to convince me that total skill points is the governing factor in capping.