Honestly, I have no idea, since I have not explored the market yet. I would think that somewhere between 10 and 20 dollars. To be fair, I am also still contemplating the idea of having the game itself be free if we will have enough funding from Patreon and the like, and just have paid accessories like soundtrack, artbook and maybe DLCs (if we get that far)
It is a completely original soundtrack. Written by this awesome guy https://www.patreon.com/FxxxKuro