Invoice Summary Sheet

I need help with a formula for an invoice summary sheet. The sheet needs to show the amount paid on each type of invoice (eg 1000, 2000 et cetera.) and also needs to be dependant to the date. I have this already:

=SUMIF('Itemized Expenses'!$A$2:$A$103,7000,'Itemized Expenses'!$E$2:$E$103)-SUMIF('Itemized Expenses'!$B$2:$B$103,"<=01/08/11",'Itemized Expenses'!$E$2:$E$103)

Changing the dates and invoice number works to an extent but as I started to input values the formula began to give values in brackets unrelated to the first few values inputed.

Cell C18 shows a value of 200 while cell C8 shows a value of 80 (two separate invoices) but all of row 18 shows the value as (80), row 8 shows all values as (200) and every cell not on row 8, row 18 column 8 or column 18 shows (280).

I'm fried on this so please help.