Big4Guy describes costs when implementing SAP or Oracle. The cost elements he describes more or less hold true when implementing any enterprise software, not just big ERP systems. Here is his list of implementation cost components (which I have paraphrased):
1. License cost
2. Legacy data migration
3. User interface customizing
4. External consultants
5. Internal project team labor
Big4Guy also correctly remarks that implementation costs are heavily driven by complexity and project scope. A larger, more complex system will be more expensive to deploy than a smaller, more constrained project.
For example, if software will be deployed inside a single department, the implementation costs will likely be lower than software deployed across an entire company. Although this is obvious, many project failures could be avoided if stakeholders were not so oblivious to that which should be obvious.