You probably also need to evaluate if the accounts that are created are fully funded or "delegated" accounts. For example, currently the truest measure of the cost of account is very small because most of the fee charged by the blockchain funds the resulting account. Practically speaking, we charge 1 steem, the blockchain charges 3 steem, but it gives back most of that as steem power in the account (maybe 2.5 steem IIRC). So real cost from our service ends up being about 1.5 steem (1 from us, .5 from the blockchain).