Do yourself a favour. Reject all software implementation estimates that you ever receive without assumptions.
Imagine going to the Builder and asking for estimates to build a house. The Builder may ask a few questions to clarify the type of house. Then, rest, he makes assumptions on your behalf and gives you an estimate stating the assumptions. For example, the house will cost you $500K; it is a standard four-bedroom premium design, with standard accessories, no tiles outside, and no reticulation. This is the baseline. Later if your requirements change, the cost will change as well. But, you know the reasons for the change in price because your requirements differ from the baseline.
Now image you request estimate for ERP implementation. The vendor provides a scope document along with the estimate of $600K. The vendor does not offer any pricing and estimation assumptions. Here you do not have any baseline against which you can trace the changes in the cost with the changing requirements. Software implementation projects are very fluid. Protect your organisation from unnecessary and unethical change requests from the implementation partners. Always have listed assumptions against the estimates. Have a solid baseline against which you can record and manage all changes.
There are no shortcuts. Each estimate must have listed estimation assumptions. If you find them missing, close your eyes and reject all estimates without assumptions!