CODuo | screen aware app that presents data

If getAmountWithTax() in your formatData method returns an instance of PhpSpec\Wrapper\Collaborator, it means that it returns a Prophecy mock builder instead of the actual mock, i.e. the one that you get by calling reveal() method. I don't know how your data provider looks like, but it seems that you're mocking your Price value objects instead of creating real instances thereof, and $product->getPrice() in your production code returns the wrong kind of object.

The solution would be either to create a real instance of the Price value object that's later returned by $product->getPrice() with new in the data provider, or by calling reveal() on that instance, like this (assuming $price is a mock object that comes from a type hinted parameter):