A gas fee on Ethereum is best thought of as a transaction fee that you as the user need to pay when interacting with the Ethereum blockchain. This fee is not set nor charged by Magic Eden, but rather is a fundamental part of how the Ethereum blockchain itself works.
The gas fee that you will be required to pay in order to send a transaction can fluctuate greatly depending on the amount of usage of the Ethereum network at the time that you are trying to execute your transaction. Gas fees can be as low as a few dollars during times of low Ethereum network usage and as high as hundreds of dollars during periods of very high demand.
Ethereum wallets such as Metamask (shown below) do a good job of calculating and displaying to you the estimated amount of gas fee you will need in order to submit your transaction. Before clicking "Confirm" on your Ethereum wallet you can take a look at the estimated gas fee and determine if you would like to follow through with the transaction or not.
Gas fees are an advanced topic and getting into the nitty gritty details of gas fees is beyond the scope of this article but if you would like to learn more about what gas fees are and how they work please consult the following articles: