The Metal wallet is linked to an Ethereum wallet under the hood. That's because Metal is an ERC20 token that resides on the Ethereum blockchain. It carries smart contract information that identifies it as MTL and needs a parent Ethereum wallet to cover for transaction fees.
$10 worth of Ethereum should enough to complete many transactions. Since the price of 1 Ether (ETH) changes daily, it's impossible to list the recommended amount in ETH.You can use an online USD to Ethereum Converter to find out how much $10 is worth in ETH.