Try it out below query, You can add more conditions as per your requirement
;WITH Sales AS
(
SELECT CostsAllocated ,
CAST(MONTH(BillDate)AS VARCHAR) +'-' +CAST (YEAR(BillDate)AS VARCHAR) AS BillDate
FROM AcFeesHistory WHERE FeeEarnerCode like 'prw' and BillDate >'2014-01-01'
)
SELECT SUM(CostsAllocated),BillDate FROM Sales Group By BillDate