Calculating Margin in CASE Statement and GROUP BY Clause
Here's one way.

select
 (select (sum(Revenue) - sum(Cost)) / sum(Revenue) from tmp where month(DateProcessed) = 1) as jan
,(select (sum(Revenue) - sum(Cost)) / sum(Revenue) from tmp where month(DateProcessed) = 2) as feb
,(select (sum(Revenue) - sum(Cost)) / sum(Revenue) from tmp where month(DateProcessed) = 3) as mar


If using the common table expression approach and more than a month is expected or included then a sum will be needed.

;with CTE_summary as (
    select
         month(DateProcessed) as monthProcessed
        ,sum(isnull(Revenue, 0.0)) as Revenuesum
        ,sum(isnull(Cost, 0.0)) as Costsum
    from tmp
    where year(dateprocessed) = 2015
    group by month(DateProcessed)
)
select
     sum(case when monthProcessed = 1 then (Revenuesum - Costsum) / Revenuesum else 0.0 end) as [jan]
    ,sum(case when monthProcessed = 2 then (Revenuesum - Costsum) / Revenuesum else 0.0 end) as [feb]
    ,sum(case when monthProcessed = 3 then (Revenuesum - Costsum) / Revenuesum else 0.0 end) as [mar]
    --,...
from CTE_summary