Marisol

Does anyone have any published pricing guidelines to help explain to a client that during development of databases, sometimes developers discover that additional items not included within the scope of the plan come up? Specifically, the client is unconfortable with a 15% contingency (that would need to be approved by the client before I proceed) built into the estimate. Resources online would be helpful, so that I can point them to an unbiased third party. Thank you in advance.