Business process to sum amounts in secondary entity and place total in parent table

How would I go about summing the costs of related attendee (table) price field for an order and then place that total back into the custom Order->AttendeePrice field. Then additionally sum Order->Total + Order->AttendeePrice into the custom field, Order->GrandTotal?

This would be similar to the Out-of-Box sum of the product sales price for an order where it places the total in the Order->Total field and manages the product total as items are added, edited, or deleted.