Two-time Most Valuable Player Curry, 29, is about to sign a $201m (£154.56m) five-year contract, his agent says.
According to Forbes, that would make him the fourth-highest paid athlete.
"Steph should be getting $400m this summer," said Cleveland Cavaliers player James, the current highest earner on $33.2m (£25m) a year.
NBA teams have a salary cap, which next season is $99m (£76m) per year [an increase from $94m (£72m)], from which they must pay the wages of all their players.
James, who has won three NBA titles, had questioned why there was a cap on players' earnings.
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Curry has helped his side to two NBA titles in the past three years and has scored more three-pointers than any other player for the past five years.
He currently earns $12m (£9m) a year and will re-sign for Golden State under the new designated veteran exception rule.
Because Curry is entering his ninth season with Golden State - and has fulfilled other criteria - he is entitled to a new lucrative deal which can be worth between 30 and 35% of the salary cap. If the reported deal goes through, Curry would be on $40m (£30m) a year.
LeBron James says Golden State Warriors point guard Steph Curry will still be underpaid despite reportedly signing the biggest deal in basketball history.