Trulia uses a completely automated system to show an opinion of value. Similar systems are used by many other sites who offer automated valuations systems. Unfortunately those systems are prone to errors and they only work with the limited information they can grab from the assessors page, etc. Since most of the upgrades, property conditions and many other special circumstances are unknown to them the valuation in many cases are not accurate. The valuation can only be as good as the data they have.
Although it is hard to comprehend how a site will value a property at half its real value.
The only way to get a good estimate is thru a manual CMA performed by a Realtor with complete MLS access or better yet thru a licensed appraiser.