It's even more expensive than GDDR3 right now because it's made in such low quantities. There aren't a lot of companies that use it, and most that do use it embedded into the die of a processor, such as Flipper.
But, if they cut the RAM in half, down to 256MB, and use the 1T-SRAM for all of the systems memory, then they could save money, and produce a more efficient machine than MS or Sony has. That is what I believe they will do.
Nintendo has different concerns than Sony or MS. Unlike the other two, Nintendo makes the vast majority of their money off of hardware and 1st party game sales. Nintendo tends to make their products very much in the classic arcade style of gameplay. They aren't complex in design, in fact many are downright simplistic, but they aren't trying to be either. It's not like Mario needs advanced AI or Mario Kart needs the physics of a Gran Turismo or Forza.
Nintendo video gaming was born in the arcade, and they've stayed very true to their roots. That design philosophy doesn't require realistic physics or graphics in their games and their hardware reflects that. It's very efficient in design, but not made to compete with the likes of Sony or MS on a pure processing power basis.