Your convert function looks fine to me...

I tried this and it worked:

DECLARE @Tmp TABLE ([Column 13] nvarchar(20))

INSERT INTO @Tmp SELECT '0.00'

select CONVERT (decimal(2,2),[Column 13]) from @Tmp

select CONVERT (decimal(2,2),[Column 13],2) from @Tmp

Have you considered using the IsNumeric() function to check whether the data is numerically convertible or not?