Format number as currency in controller

I am trying to prepare a string in a controller and that to format a
number as a currency in that string.

I can’t use number_to_currency as that only works in the view (as far as
I can see)

The kind of thing I want to do is:

mystring = "Your budget is " + number_to_currency(budget.value)

any pointers to the right direction would be most welcome