Hey there,
The thing with Endless Legend is, it doesn't specify cost for each specific unit, but rather places them into cost categories, and makes them cost accordingly. 

In SimulationDescriptors[Class].xml  you will find : 

      <!-- Unit Cost References -->

      <SimulationPropertyDescriptor Name="UnitProductionCostCheapest"         BaseValue="30"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostCheaper"          BaseValue="60"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostCheap"            BaseValue="80"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostLessThanAverage"  BaseValue="90"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostAverage"          BaseValue="110"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostMoreThanAverage"  BaseValue="120"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostExpensive"        BaseValue="140"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostMoreExpensive"    BaseValue="180"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostMostExpensive"    BaseValue="250"/>

      <SimulationPropertyDescriptor Name="UnitProductionCostGuardianKiller"   BaseValue="300"/>

And in UnitBodyDefinitions.xml you will find something like this for the cost formula: 

    <CustomCost ResourceName="Production" Instant="false">$Property(../ClassEmpire:UnitProductionCostAverage) + $Property(../ClassEmpire:UnitProductionCostAverage) * $Property(../ClassEmpire:CostIncreaseFromEra)</CustomCost>

As you can see, the ClassEmpire:UnitProductionCostAverage search for categorized values in SimulationDescriptors[Class].xml to determine the costs of units.