I'm not a programmer, but I think this is the right direction.
On your subform, create an invisible textbox. Make the textbox equal to FirstPriceField + SecondPriceField + ThirdPriceField. (Hopefully they don't all have the same name, as it appears). I believe it's:
If [Subtotal] is a calculated field in your subform, one in each record, then you should be able to put a text box = Sum([Subtotal]) in the Footer of your subform (may have to enable Form Footers).
Then as tanalee said, you can make a box on the main form to refer to that field, if you really need to.