Berserker Necklace + Obby Knife(Toktz-xil-ek) = Abyssal Whip + Amulet of Fury
Berserker Necklace(190K) + Obby Knife(90K) = 280K
Abyssal Whip(1,500K) + Amulet of Fury(2,800K) = 4,300K
Max Hit: Same (Although slightly less I believe by 1).
Cost Difference: Berserker Necklace + Obby Knife is 4,020K Cheaper. (Now that's alot)
Ony problem is whip and fury can be used separately but obby knife and berserker necklace are quite useless if you do not equip both.
So Berserker Necklace and Obby Knife is almost the same as whip and fury, so if on a tight budget I suggest going for them.
(I was not the one to discover this. It is quite well known but not used very often).
zerker necklace also lowers defense stats if i remember correctly