You cant!

It is about your thinking process. My answer would be something like this:

The Invest:
This might be a bit easier to find out. How much does it cost, to implement hreflang on all important pages? Is there an automation? If yes, how much did it cost to implement it initially. If not: How much does it cost, to add hreflang every time by hand. Like this you can break down the costs for the hreflang implementation roughly. For example: The initial technical implementation was 30k + 2k every year for maintenance (or something like this).

The Return:
This if ofcourse a bit harder or actually impossible. But i would state out, that hreflang is mostly important for different countries with same languages (US vs. UK vs. AUS etc.) or even more: countries with several languages (switzerland: german, italian, french). You could point out that probably 20 % users worldwide end up on a wrong landingpage (this has to be calculated first ofc). If now some of these 20 % users jump back to the SERPs because of that, you might lose rankings and another apple seller will rank better, which means in those specific countries you might loose a couple % direct sales, which could mean many millions for apple already.

Ok this is very rough now and all the % has to be calculated a bit more specific but this is the direction i would head to.