Usage of ”なのか” inside a sentence


During my studies I stumbled on this sentence : 彼女はそれがいくら **なの** かわかります

I am having some trouble understanding why you would need to use "なの” in this
context.

I think I understand the basics using of のです / なのです to give the sentence an
explanatory meaning, but why here ?

If we want to say : "She knows how much this costs"

Can we not just say : "彼女はそれがいくらかわかります” ? Why would we have to add なの after
いくら ?




Let's just consider the inner fragment

> それがいくらか

How would you parse this? Is it a sentence? If it is a sentence, where is the
verb?

The grammar with わかる is generally it takes a sentence followed by か to
indicate what is understood.

> それがいくらか

parses much like the English (only it potentially sounds a lot ruder in
Japanese).

> That, how much?

This phrase needs a verb. In formal speech, you might ask

> それがいくらですか

But generally you don't keep the formal verb in an embedded sentence. So, we
need to change this to an informal sentence.

You might guess

> それがいくらだか

But that just doesn't work. Instead you can opt for one of the following two:

> それがいくらであるか

> それがいくらなのか

The first of this sounds extremely literary. If you talk this way you'll sound
like someone in parliament making a speech: rather pompous.

In the second one, the な is だ transformed for use in an embedded structure.
I'm not sure quite how to explain the の but, if you're going to transform a
formal sentence ending in ですか, then

> ですか ==> なのか

So, in summary, 彼女はそれがいくらかわかります would be ungrammatical.

