There is still a cost. It's not simply the extra cable length, but also the extra labour this involves which is charged. (Excuse the pun.)
And as pointed out, no reputable contractor will utilise cable which you have supplied.
I certainly wouldn't entertain using cable supplied by a customer. They are always told "no chance" when they ask, as I cannot possibly guarantee its quality or even whether it is a legitimate product, let alone specs e.g. reactance to fire. (Dca, -s2, d2, a2 being the minimum permissible grade...