How to mod these functions? ( BigInteger )

[XADF]

Hello, has anyone a idea how to mod a function like this?

[Token(Token = "0x6000C2E")]
[Address(RVA = "0x38AF28", Offset = "0x38AF28", VA = "0x38AF28")]
[Attribute(Name = "CompilerGeneratedAttribute", RVA = "0x1CFF88", Offset = "0x1CFF88")]
public BigInteger get_UpgradeCost()
{
    return null;
}

I tried Patching and Hooking, nothing worked.