This product cost double what it did before

[October wind]

How does the word “double” work in this sentence? I’ve always thought “double” is a verb. But it doesn’t work like that here.

This product cost double what it did before.
(This is not a quote. I heard someone say it in conversation.)

 

[Rover_KE]

It’s an adverb meaning twice the amount.

 

[October wind]

Adverb! Thank you very much.