Rashi: This refers to their wealth in Torah.

R. Yonah: Yes. Their crown (Chachmah) is their wealth. "V'Chachmas ha'Misken Bezuyah u'Devarav Einam Nishma'im" (Koheles 9:15). When Chachamim succeed in wealth and honor, they are able to elevate Tzadikim and arouse Yir'as Hashem and His Avodah in the world, lower the Resha'im and flatterers, and elevate Emes.

Malbim: The crown that Chachamim wear for a sign of their Chachmah, it is their wealth. External acquisitions are not true wealth, only illusory wealth. The Kinyan of Chachmah is a true Kinyan. Chachamim are the true Ashirim, for "Yekarah Hi mi'Peninim" (3:15). Via Chachmah they separate for desires of this world and rejoice in their portion. This is the true wealth - one who does not desire and his Nefesh does not lack. The Ashir who desires and his eyes are not satisfied with wealth, he is truly an Oni.

Rashi: The ruin of Kesilim is folly - he was lazy from [engaging in] Chachmah.

R. Yonah #1: Wealth for Kesilim is their primary folly for several reasons. Then their hearts are elevated, and people stumble and honor them. This turns many people to folly, when they see the hand of Kesilus overpowers. It also elevates the horn of Resha'im, and enables them to fulfill their desires with sins. "Iveles" is repeated to strengthen the matter.

R. Yonah #2: The folly of Kesilim in their wealth is absolute folly.

R. Yonah #3: It does not say v'Iveles with the prefix Vov, for this is the reason for the beginning of the verse. Chachamim's wealth is their crown, for then the folly of Kesilim and their evil is considered absolute folly to people. When Chachamim's standing is low, the folly [of Kesilim] is not known, for many choose the Rasha's path, and this is their 'Chachmah'.

Malbim: Iveles Kesilim is guile. He knows the laws of Chachmah, just due to his desire he spreads the cover of folly over Chachmah to blind its eyes. He knows that the laws are true; the Kesil's folly and doubts are Iveles. The only thing he is sure about is Iveles!