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. This logic is used for the Ore Refinery.
You cannot have a building that costs less than the FreeUnit. If the Cost= value of the FreeUnit is higher than the cost of the building, the cost of the building will be overridden to match the cost of the vehicle. Example: Building Cost=500 FreeUnit Cost=2000 In-game, the building will actually cost 2000.
Selling the building will give you the normal refund amount (the FreeUnit cost is not taken into account), so you need to set the Soylent= for the building manually. In the case of the Ore Refinery, Soylent is set to 300 because the Ore Refinery cost is 2000 and the FreeUnit cost is 1400 (by normal rules, you get 50% refund when you sell a building).
FreeUnit and Industrial Plant logic
As far as Industrial Plant logic is concerned, 'free units' are not actually free. In the case of the Ore Refinery (Cost=2000), the game thinks you are paying for a 1400 credit Ore Miner and a 600 credit Ore Reinery at the same time. 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.