The difficulty with quantifying TPRM is that it's a preventative function. TPRM is not a dollar generator, it's a dollar protector. The real quantitative measure is what could be the potential financial impact if there were a breach or supply chain breakdown. From a data/customer perspective, if you can align a specific dollar amount impact with each data element or customer, then you can calculate what the potential impact would be with a vendor if there were a breach. As an example, say Vendor X has sensitive data for 1,000 of your customers, and the company has determined that a breach would result in $100 of loss per customer, then your potential impact is $100,000. You can do something similar from a supply chain side as well, but it would probably have to be driven more off the impact of the loss of a part/component, and how that loss impacts the production. Admittedly, I don't have a lot of experience with supply chain risks, but in either case you need to quantify the impact of a failure, and make the case that with TRPM oversight, you would help reduce the likelihood of a failure or the impact through business continuity testing. Have there been any major breaches in your industry that you could cite?
Obviously, if you're in a regulated industry that requires TPRM (i.e banking), then there is a regulatory risk also related to not having a program in place.
Good luck! Keep us posted if you figure out something that works. Adequately quantifying TPRM either to start a program or increase FTEs can be a difficult conversation for many companies.