calculating multiple fields

[wort0987654321]

i have a system that will allow a user to add four different types of extras to a booking. the problem that i am having is when there are three different extras on my report i am unable to add all of the fields toghether to get a sub total, i am using the text =[Price]*[Quantity], this brings up the total for each of the fields, when i try adding this code to the query that the report is based on i get the message saying that quantity could refer to more than one table listed on the sql clause.

is there a way around this or is the problem really easy to solve, could someone please help