I'm going to disagree with the other answers and say that a storm door will provide a very marginal insulation ability. It may not even be worth your time depending on what your actual problems are.
A storm door will protect the main door from high winds which can stop a lot of drafts if that is an issue. High winds can force cold air through the cracks of the door, but in this case, we're not actually insulating - we are providing a form of sealing. The additional physical door and the air gap will reduce air circulation.
A storm door can also help protect the door from debris and keep it cleaner. It can keep leaves, dirt, etc from accumulating on the threshold, and that can improve the life of the seal on the bottom of the door.
Cheap single pane windows on a storm door will do almost nothing for radiative heat loss. The window will feel very cold, and the window on your door will also be just as cold as it is now. A nicer double pane storm door will probably rival the cost of a new main door, so that seems like an odd choice.
If wind and a poorly sealed main door are big concerns, a storm door might be worthwhile, but for general "insulation" purposes... Probably not.
A few sources:
So, forget the storm door. Go with the caulk, spray foam, mastic, and weatherstripping first. Sealing the air leaks is the place to start.
Blocking wind is essentially mitigating forced-air leaks around the door.
However, the energy savings from storm doors are minimal. Aside from air leaks, doors aren’t a significant source of home energy loss.
Save on Energy
R value of metal storm door with single pane glass: 1.00
Colorado Energy Org
R value of 1. That's not much at all.