I would not pay dealer prices for a leather interior. Since there is not a factory option for leather from Scion most likely the dealer will be using Katzkin for the leather seats. You can always go this way and have it installed by an upholstery shop, Katzkin only sells through dealers or authorized installers. There is also Leatherseats.com or Roadwire as alternatives. They offer a bunch of different styles and options just like Katzkin but they let you choose to install it yourself. A third option is Clazzio seat covers. Unlike Katzkin, Leatherseats, and Roadwire these are actual seat covers not replacement seat material. They slip over the existing factory seat material and then hook and tie in place. They are excellent seat covers and are often mistaken for factory leather seats. Their color selection is limited but will cost you about half of what replacement seat covers will cost.
I persoanlly have had Clazzios once before and really liked them. with my current build I am going with Leatherseats most likely, just waiting on some color swatches to make sure they will work with the interior color scheme. I chose them over Roadwire due to cost, they were about $100 less, and the fact that I won't have to pay sales tax, Roadwire is in CA so I would have to pay sales tax and over Clazzios due to the color selection.
Have a look at all the options I listed above and compare pricing to see what works best for you.