The approximate amount earned from the transaction fees from the customers you have referred is listed on your Referrals page. You can get to this page by clicking on your account name and selecting Referrals from the drop down menu.
On this page the third item listed shows roughly how much you have accrued from referred users.
Because the transaction fees that those users have been paying could be in any number of different currencies, we simplify the overall amount by displaying it in Bitcoin and your preferred fiat currency.
There are a number of variables that cause fluctuation in the exact amount from moment to moment, so remember that this is an approximate amount.