If it's a newer piano, ie. less than 50 years old, and well maintained it could be worth close to what you payed for it as pianos don't tend to depreciate as much as less durable goods.  The exception may be smaller grands and uprights as they tend to be less quality in their design and construction. If it's a fairly nice piano in good condition you may be able to find comparable instruments on Ebay or for sale by online retailers.  Be aware that whatever price is listed is more than they expect to get as most people try to negotiate a lower sale price - sometimes as much as 30-50%.  The actual sale price will vary by region as well - more in a large market, less in a smaller area (like Eugene). 

Older pianos are much more dependent on the actual condition and should probably be evaluated by a technician.  There is no official piano blue book pricing for older pianos and you pretty much need to rely on the opinion of the local technician who hopefully has lots of experience with the local market.  Of course you can always ask for what you want price wise - you may get lucky - but it's also good to have a realistic idea of what's normal for your area.

Pianos built before 1900 should be especially suspect as to condition - unless they've been rebuilt by a competent professional they need all new strings and action felts at the very least and other rebuilding work is usually required for best results adding up to many thousands of dollars investment.  Without this work an older piano is not capable of playing consistently with enough subtlety of touch and tone to be considered musical in nature. Pianos from this era are also likely to be an earlier or pre-modern design - especially before 1880 - and may never reach the performance level of a more modern piano.  Most pianos have a stamped or inked serial number around the area of the tuning pins that can be cross referenced with the Pierce Piano Atlas which most serious techs should own. There are a few websites that offer to date pianos given the name and serial number - some charge a fee - this one seems good and was free at the time I looked : http://www.pianotuningmelbourne.com.au/piano-age/    They may not have all of the older manufacturers in their database but it's worth a try.