I can't get totals from transactions to display conveniently.

I have a transactions form that has subforms in continuous form view.

The main form records the contact customer or supplier, and a date for
a transaction.   The subforms work with different transaction types;
transactions with art items on one, supplimentary framing items on
another, payments on a third and so on, all on tab control pages.
I have a summary tab page that shows all the elements, but I can't
seem to get it to sum them.

I would like to be able to display an up to date total for the current
transaction record.  Everything I've tried on the form ends up showing
an error or nothing at all.

While I can get a correct sum to show up with a separate form that pops
up with just a text box total, that's an ungainly solution.

The total is the sum of ([NoOfUnits]*[Price]*[Factor]) for each element of
a transaction.

(The Factor is just a +1, 0 or -1 transaction type factor that just accounts
for whether value is outgoing, not billable, or incoming.)

Example:
2 prints sold at $100.00, a painting at $200.00
and an incoming payment of  $400.00

The summary tab page shows the details for a given transaction

2 prints x $100.00 x +1 sale			+$200
1 painting x $200.00 x +1 sale			+$200
1 check x $400.00 x -1 receipt			-$400

I want to show the $0.00 balance			=$0

The essential parts come from four tables:

1. Everything starts at a transaction table that lists client and date.
2. The item transacted, NoOfUnits and transaction type come from
	a details table matching the one transaction to many
	transaction details
3. Price comes from a table of transactable items.
4. Factor comes from a lookup table that relates transaction type to factor

Any ideas?

Charlie