It truly depends on how much you use it and maintain it for not only the expected life but capturing that difference in cost between the systems.
It is also very important that they size it properly so you actually gain the maximum efficiency while being able to remain comfortable.
To calculate "savings" you may wish to look at this blog post on my site.
I do have one other suggestion when you are considering the purchase - make sure you go with an established company with a good track record. I know plenty of Mitsubishi units are still running like a champ, while some other knock off brands seem to require repairs almost yearly if not more often.