In that case they are asking you take all the risk, so you need to factor that into the cost. You need detailed specs up front in order for this to be at all sane (developing the specs could be an independent project without obligations on either side). Depending on the uncertainty left in the spec, you need to have a multiplier on the expected costs. 5x, 10x, or even 50x would not be out of the question if there is sufficient uncertainty.

Yes this costs the company more, but if they expect you to take on their budgetary risk, then you are providing insurance, and that is potentially worth a lot more than the actual development.