Forms :: SUM Function Produces Error From Calculated Function

I have a project at hand and it's been a predecessor of mine and client has asked me to do some work on it and extend functionality - but I have not really delved into Access before and I have had to worked my way through to this final snag :/

The Main Form has one sub form. This sub form allows the user to add multiple order items i.e. qty, stock, description from records within the system - fairly straight forward.At the last column of each row is the sub total of those particular items i.e.

Qty Unit | Item ID | Total
-----------------------
2 | 1234 | 80.00
------------------------
1 | 43526 | 20.00
------------------------
> | |

So the total is a function of =[Qty Unit] * [Unit Price].Then in the Footer of this SubForm is the Sub Total

=SUM([Qty Unit] * [Unit Price])

All fine and well..... However, the additional functionality kicks in.

Lets add the additional customer_id from the Main Form. Each Item bought is dependent on the customer_id i.e. they get special prices depending on who they are.So a New table is made which has the Item ID and SpecialPriceID (of a table to define as a specialPrice) and the Price linked to this Item and Special Price category. So say that there are two groups of users "wholesale" and "nonwholesale" these would be SP_1 and SP_2 and each client is defined either one of these, and each stock item has a Price for each SP_1 and SP_2. Hopefully I've explained myself there.

Back to the SubForm. So now the Total needs to calculated differently with needed the external customer_id from the Main Form.

Code:

Function CalculateSpecialPrice(ItemID As String, CustomerID As String, Unit As Integer)
Dim SPSelect As String
SPSelect = "SELECT Price FROM [Items_SpecialPrices] WHERE"
SPSelect = SPSelect & " ItemID = '" & ItemID
SPSelect = SPSelect & "' AND SpecialPriceID = (SELECT SpecialPriceID FROM Customers WHERE customer_id = " & CustomerID & ") "

[code]....

its the sub total I just keep on getting #Error on. I have even watched (using alerts) that the correct return variable is the same as the individual rows. This is the equation I used for the SubTotal within the footer.

=SUM(CalculateSpecialPrice([Item ID], [Form]![FormName]![CustomerID], [Qty Unit]))
#Error