R. Yonah: Lips of Da'as are dearer than gold and pearls. This is when one accustom his lips not to speak until he weighs [the matter] with the scale of Da'as and Musar. This refers to a buyer who belittles what he bought while in front of the seller, even though he knows that it is worth more than he paid. This is not the way of Da'as. Why should one say Sheker and deceive the seller without benefit?! He already bought it! Sheker is despised. Before the sale, it is normal to belittle it, so the seller and buyer will agree [on a moderate price]. There is another bad Musar of belittling the item after he bought it - a seller is happy when the buyer is pleased with his purchase.

Malbim: Via exertion he acquired Sifsei Da'as, i.e. a clear knowledge of Chachmah, like something known from senses. There is a hint to this above - "Ozen Shoma'as v'Ayin Ro'ah Hashem Asah ¹ " (12). Via opening his eyes he will attain lips of Da'as, i.e. a clear knowledge. It is the most precious Kli; there is nothing like it in the world. If one bought and gave a price of gold and pearls, this is not so dear, for much gold and pearls exist. Sifsei Da'as are not common at all.