There are expenses involved in allowing a coin in to an exchange. Hot wallets and cold wallets needs to be created, and the scripts needs to be updated. If the trade volumes for a particular altcoin is high enough, then the expenses will be covered from the trade fee itself. If that is not the case, then the exchange will ask the promoters to pay up. Now this amount can vary from exchange to exchange, and smaller exchanges will obviously be ready to list coins for a smaller payment.

Once the coins are listed in one or two smaller exchanges, the next step is to inflate the trade volume using bots. And after a few weeks, the coin will gain enough trade volume to get listed in coinmarketcap. This will give enough attention for the coin and the promoters can then initiate a pump and dump cycle and vanish with whatever wealth they manage to accumulate.