Simple question, just realised i can't use my allegro cables on my sierra drums. So how much should i pay for a sierra hand brake cable and more importantly, is there a difference between the disk and drum hand brake cable?

yes there is a difference between the disc and drum cable. expect to pay £15 - £20

Dont forget that disk and non disk handbrake levers are different in the sierra.... the pivot point of the cable is closer to the axis on the disk one!