Hi Sylvia,
Giving a default value to a variable does NOT mean the object will be filled with this default value. More information on defaults you can find here: CASE: when to use placeholder, default and placeholder_default tags?
Your code needs to be changed in something like this:
{% newline %}
|Aankopen van grondstoffen en bouwmaterialen
|{{ #600 | currency }}
|{% input custom.grondstoffen.variabel as:percentage default:1 %}
|{% input custom.grondstoffen.vast as:percentage default:0 %}
{% if custom.grondstoffen.variabel == blank %}
|{{ #600*1 | currency }}
{% else %}
|{{ #600*custom.grondstoffen.variabel | currency }}
{% endif %}
{% if custom.grondstoffen.vast == blank %}
|{{ #600*0 | currency }}
{% else %}
|{{ #600*custom.grondstoffen.vast | currency }}
{% endif %}