An authentic Stam (even used) goes for more than $1k on ebay; ebayer Personalshoppers (trustworthy) recently listed several Fall '06 Stams for a little less than $1200.
Stam is MJ's most popular bag since debuting in Fall '05, it has the best resell value. The most-in-demand Stams are the discontinued colors and/or those with suede linings.