If you come across a public authority which claims that it cannot meet your FOI request because of the costs involved in redacting data, quote the following decision by the Information Tribunal (PDF) between South Yorkshire Police and the the Information Commission (Appeal Number: EA/2009/0029).
As Public Partners put it:
Under the Appropriate Limit and Fees Regulations no public authority need spend more than £600 on complying with a request, with most having a ceiling of £450 – this is called the appropriate limit. The limit is made up of the costs involved in locating, retrieving and extracting information. All the time spent on these activities can be added together at a rate of £25 per hour until the limit is reached. In this case South Yorkshire Police refused to disclose the whole of a 187 page document because redacting intelligence information line by line would cost them well over £450.
The [tribunal] ruled very firmly that the Police were wrong. Their decision couldn’t be much clearer: “we find that a public authority cannot include the time cost of redaction when estimating its costs”. All the Police’s arguments were dismissed and the rule now seems conclusive.