I dunno man I don't understand clones, doesn't it take lots of money to build a prototype of something then mass produce it, how can a copy of something be made so much cheaper. How could it be better, and how could it be legal.
Let's work backwards here.
It's legal because most (if not all) modders have no official copyright over their designs. Part of this is the fact that e-cigarettes are such a grey zone at the moment. On top of which, the clones are made in China, where lots of production and industrial rules get bent for profit.
It can be better because a lot of the clone companies get a chance to see how the original project does upon release - and most importantly it's weak points. For instance, take the Akuma mechanical mod. When it was released the button was reported as being crunchy and bad. So the cloners knew they could fix this, and nothing says "GET A CLONE!!" like reviewers saying "man the clone has a better button than the original..."
The prototyping is the only thing I don't have any real knowledge on, but if it was me I'd just buy the original, have an engineer take it apart, and tell the factory how to start mass making them. Since they didn't spend months or years on R & D they don't have to boost the price to cover the time they spent. They can sell it for super cheap as a result. Most of the materials used in these things aren't particularly expensive, and neither is the creation process. Most of these tools are used for tonnes of other applications anyway, such as flashlights, water bottles, instruments, and just about any component used in anything that has cylinders or threading.
Also again, assuming the clones come from China, they have a lot of metal deposits (from my memory of geography) so the materials don't need to be imported and cost even less. Besides, it's an untapped market. They basically have a choke hold on it since the authentic companies are only now starting to make affordable things.
TL;DR: it's doable and it's not very hard.