How much is the MetaMask fee?

Excuse me, I'm trying to understand the cost structure of MetaMask, a popular cryptocurrency wallet. Could you please clarify for me, what exactly is the MetaMask fee? Is there a standard transaction fee or are there varying fees depending on the type of transaction or the amount being transferred? Additionally, is the fee charged in cryptocurrency or does it vary by the currency being used? It would be greatly appreciated if you could provide me with a concise yet comprehensive explanation of the MetaMask fee structure.