Please take a look at the following sentence:
I didn't know how expensive the desk was!
This sentence means that the desk is very expensive, but the speaker didn't realize it then. Am I right?
You are right!
Your assumption is true for one good reason - the exclamation mark! This mark is used for 'surprise' here. This shows that the speaker is surprised looking at the price of the desk. He did not expect the price to be that high.