FreeUnit

FreeUnit and Industrial Plant logic (by Marshall)

If any building comes with a free vehicle (such as the Ore Refinery), then owning a Soviet Industrial Plant will reduce the cost of the building.
This is because the 'FreeUnit' isn't free. As far as the game is concerned, you are paying for the vehicle and the building at the same time.
In the case of the Ore Refinery, you are paying for a 1400 credit Ore Miner and a 600 credit Ore Refinery.

Further more, you cannot have a building that costs less than the FreeUnit. If the FreeUnit costs 2000 and the building costs 500, in game the building cost will actually be 2000.

Assuming the original Industrial Plant settings are used, Vehicle costs are reduced to 0.75 and building costs are not reduced.

Therefore the cost of the Ore Refinery would become:
(0.75 x 1400) + 600 = 1650

If you changed the Industrial Plant so that building costs were reduced to 0.5 as well (or made a new FactoryPlant building), then the cost of the Ore Refinery would become:
(0.75 x 1400) + (0.5 x 600) = 1350

When changing bonuses in Yuri's Revenge, don't forget that the Allied Ore Processor gives 25% ore bonus, which is sort of equivalent to a FactoryPlant with 0.75 cost multipliers for Infantry, Vehicles, Aircraft and Buildings. Yuri's Cloning Vats give an equivalent cost multiplier of 0.5 to Infantry.

Also note that multiple Industrial Plants and/or Cloning Vats stack their bonuses.
EG: Two Factory Plants for a Flak Track would result in:
0.75 x 0.75 x 500 = 281.25

I don't know if the Ore Purifier bonus can stack.

Appendix by DCoder

Considering the AI has more than one VirtualPurifiers, I would say that it stacks the same way.