This one might be a good question for your local dealer.
However, my guess is, it's going to cost an arm and a leg, maybe two... and your first born. In other words, big $$$ - and more than the original price difference and more than you expect to be reasonable. Plus, if Kia is like most manufacturers, there is no "kit" and the parts guys won't know exactly all the required pieces to make it like a stock Navi anyway (and watch them squirm when you ask).
Then again, I hope I'm wrong about this.