First split it into two - quantitative and qualitative. The first is harder but if you work out conversion rates you can then show “friction” or barriers that can be overcome via time saving ux and features (think PayPal and Apple Pay) think postcode auto fills address then you can list all design ideas and estimate how much these design enhancements might increase conversion rate and therefore attach that to increased revenue.

I do this in spreadsheets a lot for winning work and for clients at www.Univers labs.co.uk and it really helps clients understand design work is not wishy washy.

Happy to do an in-depth article at some point