The one way I know of to quantify something like "what's the value of preventing something bad from happening" is to use the kind of techniques described in Douglas Hubbard's How To Measure Anything[1]. Teach some experts within the customer company how to do calibrated probability assessment[2], then build a model, and run a Monte Carlo simulation[3] over that model. That gives you a way to at least loosely quantify things. It's not perfect, but for what you're doing it probably doesn't need to be.
[1]: https://www.amazon.com/How-Measure-Anything-Intangibles-Busi...
[2]: https://en.wikipedia.org/wiki/Calibrated_probability_assessm...