We might just be a little bias on this topic..
- How much do you trust the particular exchange you use?
- Would you rather be in control of your private keys or rely on someone else?
Every individual has there own risk tolerance. Each situation will be different. The intended use will also play a role in the ultimate decision. As a general rule 1 BTC at todays rate of $2,333.33 USD I personally feel comfortable having stored on Coinbase. Others would say $1,000 USD. Trezors' cost approx. $120 USD. If you have $500 in Bitcoin it's probably not worth it to have a Trezor. Take the money you'd spend on a Trezor and buy more Bitcoin, Litecoin or whatever crypto currency you fancy. When the times right come back and visit us to buy your Trezor Hardware Wallet!