First go into GameMechanics, so into TownBuildingSharedStats and then choose faction. So go into the building you want to change. And then you see this (dragon graveyard edited by me):

<?xml version="1.0" encoding="UTF-8"?>
<TownBuildingSharedStats ObjectRecordID="1000051">
  <NameFileRef href="/Text/Game/TownBuildings/Necromancy/Dwelling_7/Name.txt"/>
  <DescriptionFileRef href="/Text/Game/TownBuildings/Necromancy/Dwelling_7/Description.txt"/>
  <Type>TB_DWELLING_7</Type>
  <Upgrade>BLD_UPG_1</Upgrade>
  <Cost>
    <Wood>15</Wood>
    <Ore>10</Ore>
    <Mercury>10</Mercury>
    <Crystal>0</Crystal>
    <Sulfur>0</Sulfur>
    <Gem>0</Gem>
    <Gold>8000</Gold>
  </Cost>
  <ModObjectName>DragonGraveyrd</ModObjectName>
  <dependencies>
    <Item href="/GameMechanics/TownBuildingSharedStats/Necromancy/Fort/Castle.xdb#xpointer(/TownBuildingSharedStats)"/>
  </dependencies>
  <Creature>CREATURE_BONE_DRAGON</Creature>
  <DevLevelNeeded>15</DevLevelNeeded>
  <UIObjectName>Dwelling_7</UIObjectName>
  <UIStackPosition>0</UIStackPosition>
  <Icon href="/UI/TownHall/necropolis/128/d7.xdb#xpointer(/Texture)"/>
</TownBuildingSharedStats>

Copy game mechanics into data folder and take away read-only. You see the red writing over there. Change the numbers to get the building to cost like that. ex: <mercury>10</mercury> to <mercury>1000</mercury>