The Germany international still has three years to run on his current contract but has been linked with a move to Bayern Munich next summer, with Karl-Heinz Rummenigge having gone public with his belief that the player's release clause will drop to €25 million at the end of the 2014-15 season.
Dortmund have repeatedly reaffirmed their desire to hold onto Reus but Kohler believes that Bayern's bigger budget could ultimately prove decisive.
“For Dortmund it is getting harder and harder to keep Marco," the former Dortmund centre-back told Bild. "The club can only keep hold of Reus if they invest properly for the next four or five years.
"Bayern will not let up in their spending. At the end of the day, I’m sure it is a question of money."
Kohler's fellow ex-Dortmund defender, Christoph Metzelder, is also of the opinion that the Signal Iduna Park side will struggle to keep Reus happy if they cannot prove themselves capable of competing with Bayern.
“Marco has the personal ambition to win titles," he told Bild. "I believe that this is possible with Borussia Dortmund.
"But, on the other hand, of course, everyone knows that if Bayern are really serious about him, it is difficult."