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Calculate investment's interest rate to break-even insurance cost [duplicate]
You are comparing a risk-free cost with a risky return. If you can tolerate that level of risk (the ups and downs of the investment) for the chance that you'll come out ahead in the long-run, then sure, you could do that.   So the parameters to your equation would be: If you assume that the risky returns are normally distributed, then you can use normal probability tables to determine what risk level you can tolerate. To put some real numbers to it, take the average S&P 500 return of 10% and standard deviation of 18%.  Using standard normal functions, we can calculate the probability that you earn more than various interest rates: so even with a low 3% interest rate, there's roughly a 1 in 3 chance that you'll actually be worse off (the gains on your investments will be less than the interest you pay).  In any case there's a 3 in 10 chance that your investments will lose money.