According to the thread at:
http://www.priusonline.com/viewtopic.php?t=4964

The Toyota Sirius kit for the Solara also works with the Prius.  Part
numbers:  PTS31-00051 (cable), and PTS31-00050 (Receiver)

http://www.metrotpn.com/ProductDetails.aspx?ProductID=153
price on Sept. 19 2006:  $165 for receiver + $85 for fit kit ($250
total)
(Though you must call ahead if you have a 2006-2007 for more info.)

http://www.factoryinteractive.com/SiriusPrius/
(formerly http://www.siriusprius.com/ or http://www.priussirius.com/ )
price on Sept. 19 2006:  $395 with instructions, $495 with
installation, $39 for instruction DVD only