It's a lot of money for customised snoring mask (5,6)

I believe the answer is:

kings ransom

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'it's a lot of money' is the definition.
(I've seen this before)

'customised snoring mask' is the wordplay.
'snoring mask' anagrams to 'KING'S RANSOM'.
But, I am not sure how this is indicated.

'for' acts as a link.

Can you help me to learn more?

(Another definition for kings ransom that I've seen is " lot of money".)

I've seen this clue in The Guardian.

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