Borrow rate changes from day to day. You probably have to be a BD... With a favorable contract with your Clearing Firm... To do better then retail via IB... The Stock Loan business is very sophisticated these days... With borrow rates set on Stock Loan exchanges. It's a huge profit center for Wall Street. But these ETFs are complex and arcane enough... That just understanding them correctly... Can probably give you a competitive edge.
I found someone answered this a couple years ago. The borrow cost is indeed high at IB. You can borrow for free at Scottrade, but the shares often won't be available, or in a declining market you are forced to "buy-in" your position.
No interest on short stock these days. And you'll be charged for carrying these short positions. Who do you trade with?