Assessed values do not affect prices. It works the other way around: qualified sales during the period of time studied by your local assessor for a reassessment affect the final outcome.
I agree with Donna Panico that comparing assessed values and prices (asking or sales prices) is like comparing apples and oranges BUT not because assessed values for property tax purposes have little to do with market value. As Brian French pointed out they should be very close to market value. The key here is WHEN. Every valuation estimate -whether for listing price, appraised value for a lender, or assessed values - has an "effective date of valuation." As Debby Thompson pointed out an appraisal estimates the value at that moment in time. The latest assessment may have estimated the value of the land as of January 1, 2010, after considering sales that took place during the prior 12 months of 2009, or even earlier. That is a market-drive value, but only valid for assessment purposes using mass-appraisal techniques as of January 1, 2010. If you want to know the market value in October 2010 you'll have to look at more recent sales and listings.
If you want to know more see the July 17, 2010, Trulia blog posting Why Assessed Values Donâ€™t Correspond to Appraised Values, Listed Prices, and Sales Prices in the link below.