A monthly mortgage can seem like enough of a financial responsibility on its own, but there are many factors involved in home ownership that affect its fiscal feasibility. If you're in the market for a house and are wondering how your income will stack up against the rest of your expenses, here's how to determine a home cost that's reasonable for you. Determine Your Down Payment Before you start with anything else, you'll want to determine the amount of money you can put down so you can estimate your monthly payments. The traditional amount for a down payment is 20%…