Simple transaction that transfers value but have no additional data and not triggers any EVM code (the recipient is not a smart contract) consumes exactly 21000 units of gas. You just need to know the gas price to calculate the cost of the transaction.
The default gas price was 20 Gwei (20 * 10-9 ETH), but it is recommended to use lower value now. Check out ETH Gas Station service for up-to-date statistics about gas price.
You can also optimize costs by sending multiple payments in a single transaction, using a simple smart contract. Not going into the details, a single payment within such transaction will cost you ~10000 units of gas.