I think a more simple example might help:
You deposit 100,000 USD into your account.
You purchase 100,000 USD of stock -- so far, no margin is used and no interest is paid.
You purchase another 50,000 USD of stock -- you are now on margin:
Margin (loaned amount) : 50,000
Interest Rate 8% PA:
Amount You Pay Per Year: 50,000 * 0.08 = 4,000 USD
If you hold the extra 50,000 USD of stock for a year, you will pay 4,000 USD in interest to the broker each year.
However, you are charged on a per day basis, so it is important to consider how long you hold the position. If you hold it for 10 days:
50,000 * 0.08 * 10/365 = 4,000 USD * 10/365 = 109.59$ USD
So you will be paying a lot of interest... hopefully you have a good reason to hold a lot on margin with an account like that. Maybe you should consider a broker with better margin/interest rates ?