Form field doesn't POST value?

I have a form with a field (called Prijs). The value is filled depending on the choice in a select list. That is working fine, the correct value is shown in the Prijs-field and is converted to currency.
When I want to enter this value in the MAIL BODY, it will throw an error saying that it is not a numeric value. However, when I change it to {{$_POST.prijs.toNumber()}} it will show a value of 0 in the mailmessage, which obviously is not correct.
Any advice on how to get this done the correct way?