New Twiddle

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  model: [
    {
      "item_id": 1,
      "item_name":"Item 1",
      "item_price": 500
    },
    {
      "item_id": 2,
      "item_name": "Item 2",
      "item_price": 230
    },
    {
      "item_id": 3,
      "item_name": "Item 3",
      "item_price": 150
    }
  ],
  totalDue: Ember.computed('model.@each.item_price', function() {
    return this.get('model').mapBy('item_price').reduce((a, b) => a + b, 0);
  })
});

application.template.hbs

Sum is: <b>{{totalDue}}</b>
<br>
<br>

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}