Saved Search displayed in Suitelet--Amount remaining on PO to be displayed

Hi There

i am trying to create a suitelet which shows the amount remaining on a purchase order and shows the detail by project the PO line is coded to.

For example

Project	Total Vendor Bills	Total Purchase Orders
PRJ123	75000	5000

The total vendor bills figure is made up of invoice:

AAINV001, $35000

AAINV002, $40000

There is a single purchase order:

PO001, which is $80,000 against project PRJ123.

For the purposes of this suitelet, the $5,000 shown under 'Total Purchase Orders' is calculated as the Total Purchase Order coded against the project less the Total Billed against the purchase orders.

However, I cannot see a way of doing this as using the sum of purchase orders less the sum of billing amounts against the purchase orders returnings $80,000.