Let's just consider the inner fragment
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.
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
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
But that just doesn't work. Instead you can opt for one of the following two:
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.