I think it's rarely for a cex to list project for free unless the project is really worth listed. Like for example Dogecoin, I think they got listed for free on Binance due to its potential but for new projects I doubt there will an exchange to do it for free.

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So they got no choice but to pick dex. There is a project I know cryptomarketads and they resort to all free listing but the exchanges are just all dex such as forkdelta, bitcratic and etc which is technically free for those who dont know.