Celtic players earn almost twice as much as their Rangers counterparts, according to the Global Sports Salaries Survey.
The report provides an annual estimate of player salaries based on information from clubs, agents, and players.
That research suggests the average Celtic first team player earns £865,614 compared to £466,556 at Rangers.
However, over the past three seasons both clubs have increased their average wage by an almost identical amount.
Celtic's figure has risen by £147,754, while average wages at Rangers have gone up by £149,668.
Compared to where both clubs were three seasons ago, it means Rangers are up 32% and Celtic 17%.
Aberdeen facing Edinburgh challenge
Outside of the Old Firm, there has been plenty of competition among the rest of the clubs within the Premiership's top six - specifically Aberdeen and the two Edinburgh clubs.
Since Hibs were promoted back to the top flight in 2017, their average salary has increased by 16% and Hearts' has risen by 22%. However, in that same period, Aberdeen's has only gone up 3%.
To put that another way, the average wage at Pittodrie has increased by £3,997 since 2016/17 compared to £16,276 at Easter Road and £24,710 at Tynecastle.
Although Aberdeen still pay more than both clubs - an average of £138,667 per year compared to £111,600 at Hearts and £103,376 at Hibs - the gap has narrowed significantly in the last three years.
Mixed results after investment in wages
Two other clubs that catch the eye are Kilmarnock and Hamilton Academical, both of whom have seen their average wage increase by 22% and 11% respectively over the past three seasons.
Steve Clarke's side pushed into the top half last term and have increased their average wage by 12% from last season alone.
Hamilton's average has risen by £5,273 over the past three seasons, but Martin Canning's side have struggled to pull themselves away from the seasonal threat of relegation.
They currently sit precariously in 10th position despite having the ninth highest average salary in the division - £751 short of St Johnstone's annual average.