Note: I have changed the above data since I first posted. There was an error in my formula and the inventory levels actually increased. Sorry for any inconvenience.
Average sales price in Jan 2009 was $275,579 vs. $478,200* in 2008 (-42.4%)
*5 properties sold over $700,000 in 2008
Average sales price in Feb 2009 was $251,514 vs. $328,531 in 2008 (-23.4%)
Average sales price in Mar 2009 was $262.817 vs. $428,149* in 2008 (-38.6%)
*4 properties sold over $700,000 in 2008
Average sales price in Apr 2009 was $326,010 vs. $350,531 in 2008 (-7%)
Average sales price in May 2009 was $293,323 vs. $338,124 in 2008 (-13.2%)
Average sales price in Jun 2009 was $308,113 vs. $341,376 in 2008 (-9.7%)
Average sales price in Jul 2009 was $297,179 vs. $379,428 in 2008 (-21.7%)
Average sales price in Aug 2009 was $339,811 vs. $350,916 in 2008 (-3.2%)
Average sales price in Sep 2009 was $356,923* vs. $309,387 in 2008 (+15.4%).
*2 properties sold over $1,200,000 in 2009
Plymouth sales have actually decreased by 17 from the beginning of the year through the 3rd quarter of 2009 as compared to 2008, the average sales price in Plymouth Massachusetts has dropped by 18%.
One thing to take into consideration is that Plymouth is such a large town and prices can vary significantly from a small season cottage under $100,000 to a luxury home over $2,500,000 +/-. If you would like more detailed information for a specific area, let me know.

Single Family home statistics for the month of October is showing 390 current listings (23 of these are flagged with some type of contingency), with 2 under agreement, and 3 sold to date.
With the $8,000 tax credit nearing the deadline for closing on or before November 30, 2009, it will be interesting to see if sales spike in October and what impact it will have on pricing. Stay tuned.