There was a trade on that with actual numbers. Most of the time, contrary to what Batabii said, it's cheaper to buy from AH the 3* version than buy 2* and upgrade it yourself. And that doesn't always change when you already have the 2* version.

At the time I write this, a Scary Skelly part is at 18K buyout.
If you want to do it yourself you'll need :
- the recipe which is 4K from basil (6K from AH currently)
- 1K crafting cost
- 200 energy, which is either approximatively 7K + 100 mist or 14K if you don't want to spend your mist
- some mats, which have a negligible cost
- the 2* part, that I am not going to count since you already have it

Let's sum it up : if you don't use mist, crafting it yourself would cost you 19K if you find the basil at recipe, which is higher than buying it outright from AH. If you do use mist, it'll cost you 12K, and that would mean you value your mist at 6K.

I hope this answer your question.