Here is an example that operates on two attributes. It uses the attribute ID's to access the data. Follow the link provided by Scott and make sure you read the content. You need to add the attributes that contribute to the calculation as dependent attributes to make sure the script is triggered. Also, please be aware that the scripting has limitations for certain complex attribute types. Int and string type attributes work well, see Scott's link for details.

dojo.provide("org.example.workitems.providers.CalcInteger");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
dojo.require("dojo.date");
dojo.require("dojo.date.stamp");


(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;

dojo.declare("org.example.workitems.providers.CalcInteger", null, {

    getValue: function(attributeId, workItem, configuration) {

        console.log("Start..");
        var text="";
        var out = "Values:\n";
       
        // for integer there should always be a value 0 by default
        var aValue = workItem.getValue("calcInputInt1");
        text = "Value: " + aValue;
        console.log(text);

        // for integer there should always be a value 0 by default
        var aValue2 = workItem.getValue("calcinputint2");
        text = "Value2: " + aValue2;
        console.log(text);
       
        // Was: return aValue+aValue2;
        // parseInt(addMarginVal)+parseInt(costSavingVal)-parseInt(investVal);
        return parseInt(aValue)+parseInt(aValue2);
    }
});
})();