Just want to give an example: We have regression test suite of 1000 TC. Currently it take 10 people to execute it manually in 10 days. After automation of this suite, lets assume we will need 3 people to execute.

If we have a major functionality change which affects 500 of my test cases, we will need 7 people to fix scripts and re-run and update. i.e. basically maintenance of scripts. Essentially 3 fr execution and 7 for fixing. 

Where do I see saving from automation of my regression test suite

Assumption: existing scripts work perfectly on current codeline, have followed Selenium Robot framework.