What's a house worth?

A home ultimately is worth what someone will pay for it. Everything else is an estimate of value. To determine a property's value, most people turn to either an appraisal or a comparative market analysis.
An appraisal is a certified appraiser's estimate of the value of a home at a given point in time. Appraisers consider square footage, construction quality, design, floor plan, neighborhood and availability of transportation, shopping and schools. 
Appraisers also take lot size, topography, view and landscaping into account. Most appraisals cost about $300.
A comparative market analysis is a real estate broker's or agent's informal estimate of a home's market value, based on sales of comparable homes in a neighborhood. Most agents will give you a comparative market analysis for free.
You can do your own cost comparison by looking up recent sales of comparable properties in public records. These records are available at local recorder or assessor offices, through private real estate information companies or on the Internet.