My reply would be.
I'm 100% sure City have looked at designs and costs at some point. Which as you say have increased substantially.
But like you, I'm not privy to that info, so I could be wrong.
Snippets do come out every now and again from various sources.
I'd say the NS expansion isn't dead in the water, rather it's on the back burner.
The current priority is the Coop Live Arena. And maybe Joe Mercer Way and City Square, as both look very tired, and won't do the new Coop Live Arena justice when it opens.