Need help with expression builder issue

Hi all

I'm hoping someone can help with an issue I'm having. I'm creating a database for a manufacturing joinery. For 1 of my forms, Ive to create a despatch form which includes order details along with customer details. Some customers can have contracts in which they get a fixed price on certain item. This is where the problem lies.

For my customers table, I have a field called "On contract" which is a Yes/No field. I've created a query to attempt to make this form. I'm having trouble with adding a total column at the end of this query. Quantity,Unit Price, Contract Price are also in the query. I've been trying to build an expression to say like;

iif [on contract] = "Yes", [Quantity] * [Contract Price]

iif [on contract] = "no", [Quantity] * [Unit Price]

It obviously wont work, what am I doing wrong?

Thanks in advance