Changing the present value of the loan from $100k to $75k calculates a completely different payment, so you can't CUMIPMT in that way. Paying extra on a loan reduces the number of payments, so you could use NPER to calculate the number of periods with a given payment, then calculate CUMIPMT of the original loan between that period and 360 months. That amount is your savings.

However, I question the reasoning behind your goal. Calculating cumulative interest over 30 years ignores the time value of money and gives a misleadingly large amount. The present value of the interest payments should be how you compare the two scenarios.