Can I put how much I bought each piece of my collection for somewhere?

I want to sort my bills by how much I bought them for. Does this website allow you to do that, or do I just enter it into the area where I write a comment?

 Not sure about being able to sort, but yes the price can be put in comments, 

or on each line … 

https://en.numista.com/help/how-can-i-register-my-collection-on-numista-1.html 

4. A window opens, where you can enter information about your coin or banknote. All fields are optional:
· Select the version, or “undetermined” if you don't know or if it is not relevant for you.
· Select the grade.
· Enter the quantity: if you have multiple identical coin, you can enter the quantity here, or you can enter “1” and register your other coins separately.
· Enter the price at which you bought the coin [bill]. 

 and so on … 

Thanks

You can easily sort your bills by how much you bought them for by exporting to excel then sorting.