Saved millions of rupees just because of Nagios Core. Else, a Tier-III Data Center requires a paid tool for end-to-end monitoring. Even, my organization endorses me for Nagios Core with an award.

We have saved a lot of time, money, and effort in reducing disaster times, which is owed to Nagios quick alerting.

Nagios is able to minimize server downtime and this in turn helps to generate more revenue.

Can't say coz my cost of investment is only personal effort , no money so its 100% ROI

I believe it is hard to calculate for hardware.