Automate Tracking Marketing Campaign Costs from Purchase Orders & Expenses

We are working on a customized option to automate tracking of campaign expenses directly from purchase orders and expenses that are entered instead of requiring user to manually key in the expense a second time on the campaign itself.  We are adding additional fields to our POs and Expense forms to capture the campaign, but has anyone had any luck getting to the Total Cost or even Base Cost fields listed on a marketing campaign?  We want to tally up our expenses from POs and Expenses and use that number to forcefully update the Total Cost on the campaign by