If the lease-to-own period is 2 years, and the house price is $200,000. Then the downpayment should be 20% if conventional loan which is $40,000. Average to each month, it should be $1666.67. But usually this is too much.
Then if the new loan is FHA, which usually is 3% of the house price, $6000. Each month it should be: $250
Just my thought. You can also have a number that mutually agreed, but at minimum the FHA loan downpayment.