This instruction will add a new `cost` element on every call. Resulting XML will look like (with your test data):

```xml
<snapshot>
  <cost>123000</cost>
  <cost>14300</cost>
</snapshot>
```

We should use `addIf()` method, which checks whether the element exists already and adds a new one if it's absent. See http://www.xembly.org/