Pnin's glass bowl came from Victor, and is important to him for that, for the connection it represents. To others, it is valuable because it is beautiful and would cost a lot of money. Pnin doesn't really like that others think of it that way, and consequently acts gruff and tries to change the subject when his guests discuss the bowl. Instead, it is precious to him as a symbol. (Not a literary symbol, a personal symbol. Pnin's symbol, not Nabokov's. He doesn't do symbols, right? ^.~)