New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  prices: Ember.computed.map('model', i => Ember.get(i, 'price')),
  totalPrice: Ember.computed.sum('prices'),
});

controllers.index.js

import Ember from 'ember';

export default Ember.Controller.extend({
  actions: {
    add() {
      this.store.createRecord('shopping-cart-item', { price: 10 });
    }
  }
});

models.shopping-cart-item.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  price: attr('number'),
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    // peekAll instead of findAll
    // will not fetch to the server,
    // and just return whats in the 
    // local store.
    return this.store.peekAll('shopping-cart-item');
  }
});

templates.application.hbs

<h1>Totalpreis: {{totalPrice}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

templates.index.hbs

<button onclick={{action 'add'}}>add</button>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}