The 17-year-old, who has been likened to Manchester United forward Zlatan Ibrahimovic, has scored six times for his Swedish club this season.
Real Madrid, Barcelona, Liverpool and Bayern Munich have all been linked with the teenager, but Wesstrom has seemingly placed a large transfer figure on Isak's shoulders.
When asked if any deal would have to break the club's transfer record, Wesstrom told Sydsvenskan: "Absolutely. Then there is the question of what level you would be looking at. But all that you get to decide when it becomes something tangible."
Isak, who is contracted to AIK until the summer of 2018, has also scored twice in six appearances for Sweden's Under-17 team.