you should have roughly 40% as a down payment to qualify for the reverse mortgage - in some instances you will need a larger down payment upwards to 60% ( depending on your age, location, credit history and the lender you are working with )
Do a search for "purchase reverse mortgage calculator" in google and you'll find a calculator that'll provide an estimate. It'll be close, but the third party fees won't be correct. You'll need a Reverse Mortgage Loan Officer to provide you with a quote to receive an estimate with the proper fees.