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.
... Otherwise I don't think there is any threshold for how much bitcoin you own before you buy one of these hardware wallets.When I first started looking at hardware wallets 5 or so years ago... I was also of the impression that it was a bit pointless spending $60 on a device if I only had $50 worth of Bitcoin... and then Bitcoin went from $500 to $20000
Being properly prepared and setting yourself up properly to start with is a much better idea than suddenly worrying about having potentially life changing sums of money secured by a password on your desktop PC... which you also use for browsing the web, watching netflix/porn and playing games etc.