cast((cast(((iDiameter + 0.5) * 2) + 0.9999 as int))/2 as dec(18,2))

------解决方案--------------------
少了一个0.9999
SQL code
(cast((cast(iDiameter as decimal(16,2))+0.5) * 2 + 0.9999) as int)/2

------解决方案--------------------
SQL code

--是不是这个意思？
declare @iDiameter int
set @iDiameter=100
select 
cast(round((cast((2*@iDiameter+1) as decimal(18,4))+0.9999)/2,2) as decimal(18,2))
/*
101.00
*/