You have grounds for an internal review
If you feel their calculation is unreasonable, you could ask for an internal review to challenge the calculation.
Authorities may not aggregate requests unless they are all made within the same regime. Ask for an internal review, stating that they are not all FOI requests.
Authorities are advised that requests may only be aggregated if they relate “to any extent” to the same or similar information. This is a wide description which you may be able to argue against. Ask for an internal review, linking to ICO advice (PDF).
You may be able to argue against aggregation on the basis that by law, authorities must respond to requests within 20 working days. The ICO allow “the aggregation period to only run up to 20 days ‘forward’ from the date of any single request under consideration” and “up to 20 days ‘backwards’ from the date of any single request under consideration” and “the total aggregation period (running either forwards or backwards or a combination of both) from the date of any single request must not exceed 60 working days”. If the authority has included requests outside these timescales, then you should ask for an internal review, linking to ICO advice (PDF).
Ask for an internal review, stating that you are unrelated to other people who have made similar requests, and therefore their requests should not be included in your cost calculation.
Ask for an internal review, linking to ICO advice (PDF) which states that requests may not be aggregated if they were made more than 60 working days apart.
You should reply for clarification
Section 1(1) of the FOI Act requires that an authority informs you “whether it holds information of the description specified in the request”. If they have not confirmed or denied whether they hold the information, and if they have not cited the cost limit as the reason, reply to the authority, requesting clarification on this point.
It’s not a requirement, but it is considered good practice by the ICO to provide ‘arguments or evidence’ for cost calculations. Note that the FOI Act states that it is “the duty of a public authority to provide advice and assistance, so far as it would be reasonable to expect the authority to do so”. You could reply to the authority quoting this guidance and asking for more detail.
If the authority has described how they intend to conduct the search, and you think there is a cheaper way of doing so which they have not considered, you can reply to suggest it (for example, a more refined search term to use across electronic files).
Section 16 of the FOI Code of Practice advises that bodies should ‘provide advice and assistance, so far as it would be reasonable to expect the authority to do so’ and the ICO guidance interprets this as providing suggestions of this nature. You could respond to authority asking for such advice.
Refusing a request on cost grounds, but not giving you a chance to identify what information you’d like within the cost limit, is considered bad practice by the ICO, who state that the requester should be able to define which part of the information they would like to receive. Send a reply, asking for the specific information you’d prefer, and linking to ICO advice (PDF).