var v1 = (this.getField("Dropdown21").valueAsString);
if (this.getField("Dropdown21").value=="a House Person") event.value = "15000";
this.getField("C2_House_Person").value = v1
Move this code to be the custom calculation script of "C2_House_Person" and change it to this:
var v1 = this.getField("Dropdown21").valueAsString; if (v1=="a House Person") event.value = "15000"; else event.value = "0";
Make sure to set the properties of the drop-down to commit the selected value immediately (under Properties - Options), or it will only work when you exit that field.