Found this Black Book link while looking online an a 987 which an Infinity dealer has. If this pricing is correct, they say trade in value of an 05 S with 20K miles is $15K-20K. OUCH. I would be crushed if I had an 05 S, well cared for, went to trade it in and was offered $20K. It's amazing how many 05 S cars are advertised for over $30,000.
http://web1.intellipriceauto.com/etradeinbg/identify.aspx?lcdealer=6968001&Spanish=No&vLead=1&vTheme=2&vProgID=0&dealerid=6968001