Forgive if I am new to this but does that mean that inbuilt functions don't add to reported CPU ?
CPU and IO for a UDF are NOT reported by setting statistics on.
The solution that I use is to save the values from the sysprocesses table for the @@spid and then after the SQL runs, calculate the difference from sysprocesses.
You will notice that there is a significant usage of CPU when a UDF is used.
SQL = Scarcely Qualifies as a Language