I don't think "Transport Cost" has anything to do with moving supplies & tactical movement of units? It might have a correlation, since heavier units usually consume more ammo, fuel and supplies, but there are different formulas for truck usage of units.
14.2. Strategic movement
Each unit has a strategic transport cost listed in the unit detail window. For that unit to use strategic movement there must be sufficient rail capacity, transport shipping, or amphibious shipping points available to conduct the applicable type of movement. The transportation cost of a unit will be deducted from the pool of available points every turn it uses strategic movement, even if it just moves one hex.
20.1.4. Motor Pool
The motor pool represents the generic vehicles dedicated to the non-rail network portion of the supply grid. During the Unit Supply Requirements sub-segment of the Logistics phase, the computer determines the required number of vehicles that should be in the motor pool to meet supply requirements. This vehicle requirement for the supply system is based on supply and fuel usage, distance from the rail heads and the current number of organic vehicles in combat units, which increases the required number of vehicles in the motor pool by one for every two vehicles in a unit.