Why would a draft cost more to edit?
Because the work is longer per page. A draft needing heavy rewriting takes several times as long as a clean one, which is why any serious provider looks at a sample before quoting. A fixed price offered without seeing your document is either padded or is going to be renegotiated.
This comes up on How Much Proofreading Costs, where it is answered in the context of the work itself.
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