Clubs expect the salary cap to rise 15 to 20 per cent in 2028, following the NRL's $5.3 billion television deal.
Bradman Best is expected to enter free agency on November 1 after Newcastle offered $700,000 for 2028, against a projected $750,000 in 2027.
Proszenko told The Continuous Call Team Best would test the market.
"The reason he's going to the open market is because he feels 'lowballed' by Newcastle," Proszenko said.
"The offer to him to extend was only $700,000, which is what I'm hearing.
"Given the salary cap is going up... his value would only rise."
Best returned to NSW Blues this season as they claimed the series shield; their Suncorp decider win ended a long Brisbane drought in 2024.
Newcastle's existing deals may squeeze Best out. Kalyn Ponga and Dylan Brown each earn over $1 million annually, with contracts through 2030 and 2035, respectively.
Dylan Lucas and Dominic Young are contracted through 2030, while Fletcher Sharpe remains at the club through 2028.