So, reckless spin is supposed to reduce your warpath channeling cost by -6, whereas warriors fury increases it by +4 per second.
So putting one point in each I would expect a net -2 mana cost, but my mana is draining super fast since I put that point in warriors fury. My channel cost still shows as zero, I guess this is a bug ?
You misunderstood Warriors Fury, its giving its bonuses every second, so its grows stronger and more expensive each second
Fun Fact, you can make Warpath the size of the entire screen with Warriors Fury, if you spec for it.
So lets say I have 1 point in warriors fury. The tooltip suggests thats going to cost me +4 mana per second, but you’re saying it escalates for each consecutive section that you’re spinning ?
So does that mean if I am spinning constantly, the cost goes +4,+8,+12 per second ? Or something like that ?
If so, how on earth do you get the mana to sustain 5 points in warriors fury 
Yes
You don’t you use it until its empty and then you refill. If oyu have 500-600 Mana (With items like Orian’s Eye etc) you can sustain Warpath for 10+ seconds even with 5 Points in Warriros Fury.
Tools like Volatile Reversal can then Refill your 600 mana with one button press.