tsql, multiplying two columns to create a new column

I struggled through making this query.  Now I'm trying to create a new field by multiplying two fields but am not able to make it work.
This works:

SELECT Guest.RegistrationID2, Registration.RegistrationID2, StudentID, FirstName, LastName, PreferredName, StudentEmail, MailAddress1, MailAddress2, MailCity, MailState, MailZip, CommunicationPreference, IsActive, ApplicationIDPrimary, Sum(CPSTransaction.AuthorizationAmount) as Total_Paid, COUNT(Guest.GuestRegID) as Guests
FROM Registration Left Join CPSTransaction ON StudentID= ApplicationIDPrimary LEFT JOIN Guest ON Registration.RegistrationID2 = Guest.RegistrationID2
WHERE IsActive = 'True' AND (CommunicationPreference = 'Email') 
Group by Guest.GuestRegID, Guest.RegistrationID2, Registration.RegistrationID2, Registration.StudentID, CPSTransaction.AuthorizationAmount, FirstName, LastName, PreferredName, StudentEmail, MailAddress1, MailAddress2, MailCity, MailState, MailZip, CommunicationPreference, IsActive, ApplicationIDPrimary
Order by SUM(AuthorizationAmount), COUNT(Guest.GuestRegID)

When I add this line to the select statement, I break it.
SUM(CPSTransaction.AuthorizationAmount - ((55 * COUNT(Guest.GuestRegID) + 100))) as Total_Due

Error:
Cannot perform an aggregate function on an expression containing an aggregate or a subquery.


I would appreciate any help with this.
Thanks.