Everyone here is considering the supply/demand curve and the factors that will influence it. But there are still some wild cards that could have the price moving in either direction.
Affecting supply:
Roadster production limited to 2900 (or 2400) - This raise prices by limiting supply and by creating "collector" status.
Release of Model S results in Roadster owners "upgrading" - This could increase supply and decrease price.
Termination of 2-3 year leases of 2008's and 2010 Roadsters - This could increase supply and decrease price.
Affecting demand: (This is where it gets tricky.)
Other manufacturers release BEV's. This may entice would be Roadster buyers to a different car thus lowering demand and decreasing prices.
Tesla releases Model S BEV. This may entice would be Roadster buyers to Model S thus lowering demand and decreasing prices. BUT BEV's releaed by Tesla and other manufacturers will increase overall awareness of BEV's and thus increase demand.
HOWEVER, currently demand for BEV's is low now due to fears about new technology and lack of infrastructure, etc... By the time Model S is released, there will be more public education about BEV's and there will be more infrastructure. This could significantly increase the number of people in the market for a BEV. Many people considering between a new Leaf (or some other BEV) or a used Roadster may drive up demand and prices.
There are a lot of unknowns. What I can tell you is that according to my lease, Tesla is expecting my vehicle to be worth 41% of its purchase price when the lease is up Sept 2013. If they are wrong, then I'll buy it. ;-)