Lathes are normaly sold on their own, and compared to the assesories they are realitivlty inexpensive.....
There are several types of duplicators, Penn State Industries offers their own simple duplicator for $104. it's nothing elaborate, but it gets the job done...
Vega, makes the industry standard, the mini/midi lathe duplicator goes for $269. both these duplicators may be a little cheaper if you can find them on Amazon or E-bay...
Unless your making production runs, are involved in swaps or have a need for large volumes of a particular plug, your probably better off taking measurments and turning by hand....
I own both style duplicators, but unless I have a specific need for large volumes of a particular plug.... I turn with my chisels and calipers.... it's what hobby plug building is all about.
Here are pictures of both style duplicatoors, first is the PennState industries, the second is of the vega....