HFF is right. What is the house worth today? That is the price you work with. If you paid $200,000 and it is now worth $300,000, you would owe her half the difference between $300,000 and the $100,000 mortgage. If the value has slipped to $85,000, you don't owe her anything. But things get negotiated, and you will be asking her to sign away her rights to the house, so you probably would have to pay her something.
