Actually, it's fairly simple. If the house is listed above $550/$600k that means most people who are looking at buying it have to qualify for a jumbo loan (harder to get, require larger down payment, etc...) In that territory, sellers are being forced to be more flexible. In that case, you can use a site like zillow to determine roughly how much prices have appreciated over the last 10 years. Take that level of appreciation and knock the current price back to about 2003 or 2004 and you are in a rough ballpark for fair value. I personally think the market will have to go back to 2000, 2001 in Boston, but in Lexington no one will sell at those levels today.
Under $600k, it gets a little more complex because the inventory is tighter, loans are less expensive, and an increasing number of people begin to qualify for tax credits. All of this serves to buoy prices. I've also noticed sellers who bought prior to 2002 are generally significantly more flexible on pricing over those who bought between 2004 and 2007 as the earlier ones are still getting abnormal returns and the later ones will take a hit.