FreeUnit

FreeUnit= is a building tag that takes a value from the VehicleTypes list to tell the game that this unit should appear beside the building when it is constructed. The cost of the free unit is actually included in the cost of the building so selling the building will only give you (cost of building - cost of unit)/2. This logic is mainly used for the Ore Refinery, where 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 and the building cost will actually be taken as being 0.

FreeUnit and Industrial Plant logic

As discussed, FreeUnit= cost is taken into account when calculating the cost of a building.

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

Similarly, the Ore_Purifier bonus stacks so that building more ore purifiers results in more credits per bail returned by the ore miner.