Calculate Subtotals using fixed categories in postgresql query

Question

I have a query that returns the count and sum of certain fields on my tables, and also a total. It goes like this:

---

example:

with foo as(
select s.subcat as subcategory, 
       sum(s.cost) as costs,
       round(((sum(s.cost) / (s.tl)::numeric )*100),2)|| ' %' as pct_cost
       from (select ...big query)s group by s.subcat
      )
select * from foo
union
select 'Total costs' as subcategory,
       sum(costs) as costs,
       null as pct_cost
from foo
order by...

Category	Cost	Percentage
x_subcategory 1	5	0.5%
x_subcategory 2	1	0.1%
x_subcategory 3	18	1.8%
y_subcategory 1	7	0.7%
y_subcategory 2	10	1.0%
...	...	...
Total	41	5.8%

And what I need to do for another report is to get the totals by Category. I have to assign these categories based on the value of the subcategory name, the point is how to partition the result so I can get something like this:

---

Category	Cost	Percentage
x_subcategory 1	5	0.5%
x_subcategory 2	1	0.1%
x_subcategory 3	18	1.8%
X category	24	2.4%
y_subcategory 1	7	0.7%
y_subcategory 2	10	1.0%
Y category	17	1.7%

---

With GROUP BY and GROUP BY GROUPING SET I don't get what I want, and with PARTITION I'm getting syntax errors, I'm able to use it in simpler queries but this one turned out to be very complicated and I wonder if it's possible to achieve this on a query on PostgreSQL.