Final, 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')
,('x3', 20,'2020-05-14T01:02:00')
,('x2', 30,'2020-05-15T03:12:02')
;
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
      ,ROW_NUMBER() over (partition by label order by time DESC) as r_number2
    ,RANK() over (partition by cost order by time asc) as TEST
    ,Row_NUMBER() over (partition by label order by TIME DESC) as TEST2
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
      ,ROW_NUMBER() over (partition by label order by time DESC) as r_number2
    ,RANK() over (partition by cost order by time asc) as TEST
    ,Row_NUMBER() over (partition by label order by TIME DESC) as TEST2
       from Table1
     )
DELETE from todelete 
    where (cnt > 1 and r_number between 1 and (cnt/2)*2 and cost=ISNULL(lastcost,cost) AND TEST2 !=1)  OR (cnt>1 AND TEST2<>1 AND r_number2 != 1)
    
 SELECT * FROM Table1

View schema
Execution time: 0,03 sec, rows selected: 5, rows affected: 4, absolute service time: 0,19 sec, absolute service time: 0,19 sec