This is an SDK mod to allow buildings to modify the cost of upgrading units. The mod as-is includes a Leonardo's Workshop wonder, working as it did in Civ III, halving the cost of upgrading all units. Given that the mod exposes the UpgradeCost modifier in CIV4BuildingInfos.xml however, you could make that -50% 
anything you wanted, from -1% to -100%.
Original credit for this mod goes to Impaler[WRG] for writing the SDK, I believe for Warlords. Although I can't find his original upload anymore, his name is sprawled all over the SDK changes, so I know it was him!
The artwork was originally created by GIR, AFAIK. I honestly don't recall if I simply preferred his over what Imapler had originally used, or if Impaler was using GIR's work. Sorry...
If I've missed someone, please don't kill me.
I've had this mod for absolute ages, and recently updated it to BtS v3.19, so I decided, "While I'm at it, why don't I get this out there for everyone else?"
So I did.
anything you wanted, from -1% to -100%.Original credit for this mod goes to Impaler[WRG] for writing the SDK, I believe for Warlords. Although I can't find his original upload anymore, his name is sprawled all over the SDK changes, so I know it was him!
The artwork was originally created by GIR, AFAIK. I honestly don't recall if I simply preferred his over what Imapler had originally used, or if Impaler was using GIR's work. Sorry...
If I've missed someone, please don't kill me.
I've had this mod for absolute ages, and recently updated it to BtS v3.19, so I decided, "While I'm at it, why don't I get this out there for everyone else?"So I did.
- INSTRUCTIONS
- Simply unRAR the file into your BtS\Mods directory.
- VERSION
- I honestly don't know what version # Impaler[WRG] had gotten to, so let's just call it version 2.0, since it did need to be updated to be BtS compatible.
- INCLUDED
- The Upgrade Cost Modifier DLL.
- XML & Art files for the Leonardo's Workshop Wonder, giving -50% to unit upgrade costs.
- The changed source code files.