Test1, Sql Server - rextester

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create table Table1 (label nvarchar(max),  cost int,   time datetime);
insert into Table1 values 
('x2', 29,'2020-05-14T01:00:00')
,('x2', 29,'2020-05-15T02:12:02');

SELECT label, cost, time
           , count(*) over (partition by  label) as cnt 
           ,lag(cost) over (partition by  label order by time ASC) as lastcost
           ,ROW_NUMBER() over (partition by label order by time ASC) as r_number
           ,DENSE_RANK() over (partition by time order by label asc) as d_num
           
           FROM Table1

; with todelete as (
      select *, 
           count(*) over (partition by label) as cnt, 
           lag(cost) over (partition by label order by time ASC) as lastcost,
           ROW_NUMBER() over (partition by label order by time ASC) as r_number
           ,DENSE_RANK() over (partition by label order by time asc) as d_num
      from Table1
     )
Select *  from todelete 
    where (cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost))

SELECT * FROM Table1

View schema
Execution time: 0,02 sec, rows selected: 6, rows affected: 2, absolute service time: 0,19 sec, absolute service time: 0,19 sec