Number Formatting in ObjectListItem

Hi

Maybe a basic question from someone new...

I have create a List using ObjectListItem to present a Month with an associated value, the data coming from a JSON file.

<ObjectListItem

     title="{income>Month}"

     number="{income>Cost}"

     numberUnit="EUR"

     >

This works as expected, except that I don't seem to be able to format the Number.

How would I do that?

I am using XML Views (if that makes a difference).

Thanks

Al.