Yeah a margin balance of $1,000 comes out to around 20 cents per day it's being used at the 7.20% rate.
balance x 7.20% (or .072) divided by 365 = .197 (or $0.20) per day
So you make a 5-day trade using $1,000 margin, you pay them around $1
That is the answer I received from customer service a few weeks back.