Sydney Roosters centre Robert Toia chose Tonga over the Kangaroos, despite Australian interest.
Australia's players receive a flat $40,000 under a deal between the Rugby League Players Association and Australian Rugby League Commission.
Players from other nations earn $850 per match, capped at $4,250 if their team reaches the final.
Dolphins forward Felise Kaufusi, a Tonga representative omitted from its World Cup squad, says he will pursue fairer payments.
Appointed the RLPA's general president in November last year, Kaufusi plans to raise the issue in the next CBA.
"I want more equality in the prize money. It is something we will raise in the next CBA," Kaufusi told AAP.
He added: "Australia is always going to get first preference, which is fine, but we need to get to a stage where it is a lot more even."
Before squads were named, several national coaches warned AAP that Australia's deal could tempt players to choose the Kangaroos for financial reasons.
Kaufusi said most players prioritise heritage and culture over money, but fairer payments could remove the financial incentive.
In 2017, Tonga captain Jason Taumalolo chose Tonga over New Zealand, while former Australia prop Andrew Fifita also joined Tonga.
Both gave up the $20,000 then offered to players from the two tier-one nations.