I just bought a Prius, and want to make it look good, so I got on craigslist and picked up a set of 19 inch trd rims off of a 2010 xb, the diameter looks the same, and they seem pretty light I mean, they are trd's, just wondering if its ok, and how bad it will hurt my mpg
The 2010 XB has a 5 x 114.3 bolt pattern and your 2012 Prius has a 5 x 100 bolt pattern. The TRD wheels won't fit your car unless you install wheel adaptors. You may also have clearance issues (wheel may stick out too far) as well.
Like Dakine stated above and you will lose 4+mpg depending on tire choice. My personal guess is a 6mpg loss with a 19" wheel. If they do fit after you get the adapters, you will need to lower it or it's going to look like a donk IMO.