Using your numbers, how about 3rd-ing the spread.
2008 - 50,000 to 55,600
2009 - 55,600 to 61,200
2010 - 61,200 to 66,800
But it all amounts to the old Theory vs. reality.
In theory, if you spend crazy hours researching prices, options, high/low, yada, yada, yada, you can get a "good deal", on...