What is the cheapest way to send ether to 10 addresses? By using smart contract or using externally owned account to send one by one...? I know that the gas limit of a normal transaction is 21,000...so 10 of this transaction costs 210,000 gas. I think send ether to 10 addresses by using smart contract has cost more than 210,000 gas, correct?
10 transactions from an EOA will cost 210,000 gas.
The most efficient way to do this would be to deploy a contract that accepts funds in the constructor and subsequently distributes the funds to 10 different addresses. This will cost the initial 21,000 gas, plus the cost of the transfers, which is maybe ~100,000.