Hi,
I have the following stamp:

I can identify the stamp itself, but I cannot find a reference to what the 21,00 means at the top. Is that the cost of an entire sheet which just happens to be attached to this stamp or is it something else?
I have read that the stamp is relatively rare if used, but this one is clearly not used and it is also definitely not mint. I'm not sure how to value this stamp any help would be appreciated. I'm not looking for an exact amount, I'm trying to figure out the amount I should insure an entire collection and I'm not sure if this stamp has value that should be considered or is essentially worthless (from an insurance point of view).
I am still getting used to scanning and uploading pictures, so I'm sorry this one is skewed. If that is an issue, I will try again and repost the image.
Thank you!