The Eredivisie side are keen to lure the young attacker back to his homeland, but will not make any hasty decisions as they refuse to overspend
The Eredivisie side are keen to sign the 19-year-old to replace Luuk de Jong, who left the club for Borussia Monchengladbach, but they have so far been unable to agree terms with Inter over Castaignos.
"There's still a pretty big gap between what we want to pay and what they're asking. We'll have to think long and hard about it," Munsterman told De Telegraaf.
"I won't be rushed into making a decision. That will only cost the club a lot of money. This transfer could take a while, because Inter are clearly not in a rush either.
"This kind of things always take a while. We were also locked in talks [with Gladbach] over Luuk de Jong for up to three weeks."
Castaignos joined Inter from Feyenoord in the summer of 2011, but has struggled to live up to the high expectations.