I'm sure there'll be commercial reasons volume/buying power in price variations.
Do bear in mind that there are Ducati ones and Ducati ones out there, same as there's fake bearings and fake spark plugs etc.
Price isn't always a guarantee of quality though, buy from a reputable dealer and preferably one where you can see the whites of their eyes if you need to.
LCGB Club Shop sell's then out at £32.50p, to give you an idea of the price you should be expecting to pay for a genuine boxed Ducati CDI Unit. (If they are not in a box or packaged as genuine Piaggio replacement items, I would be wary).
Agree with rossclark that buying in (significant) bulk does bring down the price some what.