Why is Cost of Capital great in theory, but difficult in practice? If I expect to live 30 years after retiring at age 60 and I am 25 right now. I anticipate that I will need approximately $75,000 per year for my retirement. If I can earn 8% per annum return (a.) how much will I need at age 60 to purchase the 30 year annuity that pays $75,000 per year and (b.) how much will I need to put aside each year to have enough money to buy that annuity.