Well, there's this (preorder) item that does universal audio and video....
http://www.coastaletech.com/viewtech_2005.htm

Here's another brand of adapter for certain Jenson stereos... (the price looks high, but the stereo is bundled with the adapter)
http://mywebpages.comcast.net/bumpelo/mp3_player.html

Here's the google search that found it, with additional hits
http://www.google.com/search?q=Prius...en-US:official

In general, when you ask a question like this, someone will type a few keywords from your question into google, and that will be the normal quality of answer. You could go to google.com and punch a few queries in for yourself. You'll have to wade through a lot of hits about "Toyotas except Prius", I'm afraid, because Prius adapters are, well, "extra-special".

The problem is (as far as I was able to discern while looking at buying the Prius), you'll need some form of microcontroller to talk to the Prius and get it to accept the signal inputs from your device so it can play through through the Prius audio system (said to be very adequate) that is totally integrated with the chimes and GPS map and whatever else.

An FM modulator would work, if you didn't mind the signal degredation, and (typical) reduction in antenna sensitivity, but I'm afraid you're not going to drive the Prius speakers your own radio's amp without cutting wires and doing brain surgery (that is, unless you don't mind disabling the Prius' multitude of built in noise makers... then it's just cutting speaker wires and splicing them to your radio).

In short, you're probably not going to find something less expensive than a whole new car radio that will do the job right.