0.5% should be the minimum fee for a single bitcoin transaction. This is because with the current fiat transaction fees I pay which is around 1-2%, 0.5% should be justifiable because there is already a network which provides all the necessary mechanism for generating a transaction and a company will just have to make sure that the account's private keys are secured
I think the ideal is 0.5% for each transaction to be cheaper than bank transactions. Transactions using bitcoin are cheaper, because transactions that occur between wallet owners and recipients are very effective and efficient because there are not too many rules. The security factor must be taken care of themselves because the wallet contains bitcoin, the private key is only the owner who knows.