There is salvia growing all over really, it doesnt require any looking after to grow, so you might think its a lot. But I think its expensive to make it stronger and you have to know what your doing which is why it costs a much. That or wellcoolstuff just rips you off.
Common salvia (salvia officinalis) grows all over, yes. It's one of the simplest plants to grow. Salvia divinorum, however, does take quite a bit of care. It's a fragile plant, extremely sensitive to its environment, and requires a lot of initial humidity when grown from a commercial cutting. They are not the same subspecies. Salvia divinorum has been grown from clones for a very long time (centuries, if not millenia), and thus takes a green thumb to grow since it is not as hardy as its garden-variety cousin.
And you're right. Because it is a standardised extract, it is more expensive; it is several times the relative strength of the original leaf. But the primary reason is because of commercialism; the plant is in high demand, and thus, they mark up the price quite a bit.
Limpbiskit66, if you have specific questions on the extraction process, PM me. I'd be more than happy to help you, but what you asked is a bit broad. Basically, you want to utilize a polar and non-polar extraction (which will extract various parts of the plant, thusly purifying it in the process). Most salvia extractions require very little expertise, and are simpler than you'd think. I wouldn't recommend acetone as a solvent though.
I would, however, recommend a good fortified leaf over a pure extract, since you'll need something to smoke it on anyway. This obviously depends on how much leaf you'll be receiving, as well as the quality of it.
~Ethno