I have GT with signal side mirror.
However, I modified my wife's Prius and installed signal mirror. Assuming that there is not much difference in design on this, I can tell you what was involved in my mod of Prius.
No. The mirror was not pre-wired.
Running the wires thru the rubber boot (connecting the door and body) was the most challenging part of the installation - some grease or oil will help a lot!
Electrically it was very simple. Connect the red wire (power) to signal light (or equivalent - if not found - run it all the way to the tail light - use a signal tracer or get a wiring diagram would also help). Connect the black wire (ground) to any grounding point I could find. (Hint: there should usually be one inside the kick panel next to foot well.)
Pricing-wise, it will be costly if you want to buy the entire set of signal mirrors and throw away the current ones. With my Prius, there was a guy selling side mirror "cover" only. The covers came with LED signal lights. Therefore, I kept the old side mirror base and popped the old cover and replace it with LED cover. The cost was much less (around $100 if I recall correctly). As I said, running the wire is difficult if you don't enjoy it. The space is tight if you don't have the right tool to run the wires thru the rubber boots.
Send me PM if you want to know more.
It took me 2-3 hours for both sides. Trying to overcoming the rubber boot on the 1st door takes time.
Once you get the hang of it, the second one comes naturally.