On an old, large project with technical debt how can you reliably estimate or measure the benefit of refactoring code?

For example, say you have some components within a software stack solution written in an older language, and some later components written in a newer language. New features and bug fixes are constantly being added to this solution by a development team.

The developers suggest that replacing the existing old language components with the new version would be 'a good thing' However, no extra features would be added by this work and it will cost x dev-days.

A sales person suggests adding 'a great new feature'. the company measures the value of his suggestion by using a formula. ie. It will earn the company F million pounds, over t years at a cost of x dev-days work

How can the company cost up the refactoring proposal in a meaningful way? and under what circumstances is it likely to be the most profitable option for the company?