First, CAST as VARCHAR:
LTRIM(RTRIM(CAST(CAST('2.50823E+12' AS FLOAT) AS NVARCHAR)))
Second, CAST as MONEY:
LTRIM(RTRIM(CAST(CAST(CAST('2.50823E+12' AS FLOAT) AS NVARCHAR)AS MONEY)))
Now, let's see this in a CASE statement:
,CASE
WHEN [AMOUNT] LIKE '%E-%' THEN LTRIM(RTRIM(CAST(CAST(CAST([AMOUNT]AS FLOAT) AS NVARCHAR)AS MONEY)))
WHEN [AMOUNT] = ' ' THEN NULL
ELSE [AMOUNT]
END AS [AMOUNT]