Filter by category

Hello,
How difficult would it be to add an additional ability to filter to a specific category of articles?

I have two sets of content that use the same tags, but only want to filter one category of articles and only provide those articles form that category in the filtered results. As I can see it currently searches the entire site categories and articles and displays all articles as the result.

I only want results from one category.

Is this possible? Would there be a cost for customising your module? and if so how much?

Thanks