The tax could be flexible and be based on the average of all the team's player expenditures. The 3 teams which exceed the average, subsidize the 3 teams with the lowest payroll, dollar for dollar. The two teams in the middle would form the upper and lower average.
Ideally the subsidy should be shared equally with all 3 lowest payroll teams so as not to encourage any team to massively underspend, then be subsidized to the average. Also no team should be subsidized to exceed the average payroll.
This way if a team wanted to keep a valued veteran for $150k, instead of replacing him with a cheaper player for $70k to fit under the cap, could pay him his salary, then chip another $80K into the subsidy kitty (as his salary could put them $80k over the average)