This is the 296th most frequent Czech word.
peněz
"Peněz" is the genitive plural form of "peníze," meaning "money" in English.
Here, 'peněz' is used to indicate a specific quantity of an uncountable resource (money) and appears in the genitive plural due to its relation with the word 'mnoho' (many).
Utratil jsem mnoho peněz za knihy.
I spent a lot of money on books.
The usage of 'peněz' ties to describing the contents of the bag, where it represents a quantity of money and is governed by the adjective 'plnou' (full of), needing the genitive case.
Viděl jsem tašku plnou peněz.
I saw a bag full of money.
The word 'peněz' in this sentence refers to 'money' in the sense of lacking sufficient funds and is used in the genitive case to indicate the amount of a countable or uncountable substance.