You should probably go through your normal bidding process...start from the beginning (materials, labor....everything). From there I would recommend you take off 25% - 35% off of your total estimate or just the WHOLESALE value of the materials onsite, and give the client that price. There is no way that you're not going to have to purchase some materials for the project. It's really hard to know what has actually been done by the other contractor....so you're going to need a little extra room to make changes.
Design - Build - Restore