A property is only worth what a ready, willing and able buyer is willing to pay.  You can try to determine what your property is worth by looking at the sales price of other properties around you, how homes currently on the market are priced, and what comparable homes are selling for in your area.

There are Internet programs that will give you approximate values and others that will give you an on-line appraisal.  RealEstate.com provides free assistance and Case Schiller & Weiss will sell you an appraisal on-line in minutes. 
Another way to determine value is to hire a local appraiser who is on the approved list of several area lenders.  This way you should get a professional, written analysis and most likely the buyer can have their lender use that same appraisal and spare them the cost.   Remember, a home priced well will sell quickly.

You may also engage a real estate agent to give you a free Comparative Market Analysis.  A well done CMA will reflect recent sales, homes currently on the market as well as expired listings.  This will give you a broad picture of the current market situation within the immediate proximity of your home.