It depends on the exchange. some exchange give reasonable dust token. exchanges like binance and kucoin. the rest gives nothing and they worth Nothing. i was lucky to sellout during the last bull run and made a little 100usd. the money has been utilize anyway for another token. i think Dust are rewards depending on the left over in your exchange wallet.
It is quite like that. I noticed this myself but I didn't bother to get it out after trading your coin/token to whatever crypto you exchanged it to. Unlike other exchanges where they accept even the smallest amount in trading and then you'll be left out with nothing. In other term, dust also refers to an amount where id sent to your wallet which is explained in post above.