You should have taken the chance when you had the opportunity, the fees are not Binance call to make, it may go up or may comes down but the 8mbtc they are charging users is high in my opinion. I'm very sure they will be sending you one from a single input and since you are sending to external wallet, it shouldn't cost more than half of that amount but I forgot that Binance are after your fees to survive as a business platform.

What you should do is maybe you should take out Rune from Binance, make use of the Dex and then convert it From Rune to typical bitcoin on the exchange but the disadvantage of this is that even me, I don't know if the fees might cost you more than what you are trying to avoid here on Binance, so I think you should check the fees before trying out my option.