Very quick one....there are several ways....one way (probably not the best for performance, but...)

SUM ( case when Category = 'Capacity' then Total_Cost else 0 end) as Whatever



Again, not great for performance, but I believe it should work.

Alternatively, if that doesn't work, you could do a subquery with the total just for that category, and then join to the subcategory results.

But let me know if that works...