If I were to suggest an amount of Bitcoin you're holding starting from which purchasing a Trezor is worth it, it would probably be $500.
Hardware wallets are mostly good for those people who are looking for convenience and want to have a good security because they either can't or don't have the time to learn how to do things on their own. Otherwise I don't think there is any threshold for how much bitcoin you own before you buy one of these hardware wallets.