How to group query by x unit

try

select  case when NbPage between 1 and 500 then '1-500'
                       when NbPage between 501 and 1000 then '501-1000'
                        end + ' Copies' , count(*)
from (
 SELECT Sum(PageCount) as NbPage, UserLogon FROM AuditTrail
 where CostCentreName = 'Etudiant'
 and Timestamp >= convert(datetime, '2011/04/13')
 group by UserLogon order by NbPage desc
) t
group by case when NbPage between 1 and 500 then '1-500'
                       when NbPage between 501 and 1000 then '501-1000'
                        end