WTF? FRR for BTC is only 0.0155% daily. That's about 5% per year after fees.
Look at the bright side, its very cheap to short bitcoins! and we are in a down trend...
Considering that bitcoin inflation is still about 10% per year, you are indeed paying a negative real interest for them. It would be interesting if you could then withdraw the BTC that you short on Bitfinex.