format the rate in the plan model

assets/javascripts/discourse/models/plan.js.es6

@@ -1,6 +1,13 @@
+import computed from "ember-addons/ember-computed-decorators";
 import { ajax } from "discourse/lib/ajax";
 
-const Plan = Discourse.Model.extend({});
+const Plan = Discourse.Model.extend({
+  @computed("amount", "currency", "interval")
+  subscriptionRate(amount, currency, interval) {
+    const dollars = parseFloat(amount / 100).toFixed(2);
+    return `$${dollars} ${currency.toUpperCase()} / ${interval}`;
+  },
+});
 
 Plan.reopenClass({
   findAll() {

test/javascripts/models/plan-test.js.es6

@@ -0,0 +1,17 @@
+import Plan from "discourse/plugins/discourse-patrons/discourse/models/plan";
+
+QUnit.module("discourse-patrons:model:plan");
+
+QUnit.test("subscriptionRate", assert => {
+  const plan = Plan.create({
+    amount: 2399,
+    currency: 'aud',
+    interval: 'month'
+  });
+
+  assert.equal(
+    plan.get("subscriptionRate"),
+    "$23.99 AUD / month",
+    "it should return the formatted subscription rate"
+  );
+});