The machine ability of magnesium is excellent. The problems you face are not in fabricating the center, but rather in making it stiff. The young's modulus and ultimate strength of MAG is quite low when compared to other engineering metals. It is like engineering with silly-putty. You are going to want to spend plenty of time analyzing your designs so that they are stiff and light. It is useless to have suspension with excellent camber control and tow control, just to have wheel centers flexing all over the place.
We spent ~ 55$ per shell, and got the material for the centers donated.
The nice thing about he three piece wheel is it's versatility. You can change wheel width and offset, and keep the same centers. This means that you can have more design options for less $$$. Since the wheels are modular, they can easily be repaired. If you strike an object that damages a shell you can change that shell and not junk the whole wheel. Also the aluminum shells can be bent back into proper shape(assuming the amount that they are out of shape is small, say less then 3/8 on an inch) The ability of the spun wheels to deform rather then crack also means that they can absorb a lot of energy in the event of an accident. Of curse caring around all the nut and bolts on your 3 piece wheels is a trade off.
All magnesium wheels are almost always cast and then machined.
The cast magnesium wheels will crack after a much smaller amount of deformation. This means it is far less like that you will be able to repair a damaged rim. The magnesium wheels also absorb less energy in a accident then aluminum due to there more brittle nature.