Option Price to extend seller financing

What is a reasonable amount to put in a contract to be able execute an option to extend the seller financing an additional two years from (7 years to 9 years - payable at the end of the 7 years)? The outstanding balance at the end of 7 years would be about $100K.

Do you ever put terms like the market interest rate has to be over x% or buyer has to try to find financing in order to execute the option?