Is it possible to create an expression that sums up values from a number of records that may not be created yet?

I have a main form that identifies a piece of equipment and on each form are three subforms: one for maintenance, one for service and one for the totals.

Each time a piece of equipment is serviced, the costs for various things for that service period are entered in a new record. I would like the totals page to take the values that are entered on each record, and update that particular total.

Is this possible? I'm very new to Access (2K) and still learning VBA.