The answer to your question depends heavily upon how complex your drawings are (whether or not you have to check the interfaces between parts, tolerance requirements, interchangability requirements with standard or custom parts, etc., etc.); how extensive the changes to the design are; whether you are working with your own drawings or someone else's; and how you produce drawings (software or hand drafted).
For a very rough order or magnitude estimate you could take some percentage of the time to draw, check, and approve. What that percentage to use is open to considerable interpretation and judgement but, something like 15-25% may be an acceptable answer to your client. Of course, experience is the best basis for estimate.
As for how to accomplish an ECP I suggest you consult ASME Standard Y14.35, Revision of Engineering Drawings and Associated Documents (about $50 from ASME.org). Another very good source of guidance for drawings for the government is the Drawing Requirements Manual by Global Engineering Documents (about $200, over 1,000 pages from global.ihs.com).