Hi

It would depend on the type of scale you are looking to make it out of.
Various scale-associated hides and scales exist. In order of least expensive to most the 'hides' go: drakescale (teir 3), young dragon scale (teir 4) and then sea serpent scale (teir 5). You've also got ancient dragon scale that you cannot buy and must quest for the materials for.

For torso armour how much are you looking to cover? The whole back and front?

If so the basic costs would be around:

Drakescale - 675 GP
Young dragon scale - 900 GP
Sea serpent scale - 1350 GP

Bazaar note: these are made with the material size being akin to leather as they are all hides.