Dortmund sold Aubameyang to Arsenal in January, and are focusing their efforts on recruiting another centre-forward this summer.
Morata endured a difficult debut season at Chelsea following his move from Juventus, scoring 15 goals in all competitions in 48 appearances.
Zorc believes it will be incredibly hard for Dortmund to extract the Spaniard from Stamford Bridge after he cost an initial £58 million ($76.8m) last summer.
"I would say that Morata is too expensive - without knowing exact sums," he told Bild.
His statement on Morata feeds into a wider frustration regarding the state of the current transfer market, with fees as inflated as they have ever been.
And Zorc admits that he and Dortmund would have rather kept Aubameyang than be forced to find a replacement.
"Of course, we have candidates in mind," he said, when asked about replacing the Gabon international.
"But the position of a true centre-forward is currently the most difficult to fill in European football. That's why we've resisted selling Pierre-Emerick Aubameyang for a very long time because he guaranteed us goals.
"At the moment I have the impression that the transfer market - not only in the case of strikers - is a bit out of touch with the realities.
"Very high prices are called. We have to act with caution and not do crazy things. Especially as we have players in our squad who also have this position in their portfolio."