Seems like there's a table missing from your description unless you only order a single product on any one order record. Or does each record in product represent an ordered item instead of a "catalog" of items to be ordered?
summary fields evaluate different totals and sub totals depending the context in which they are used.
Say that you have this relationship:
Orders::__pkOrderID = Products::_fkOrderID
If you define a summary field named sProductTotal in Products to compute the total of Products::Cost, then you can place Products::ProductTotal on an Orders layout and it will show the total for all related records in Products linked to the current record on your Orders layout.
If this is what you have done and you are not getting the expected total, then you need to check the relationship used to link orders to products. (Note: Let me know if you are using a portal filter as this will change the details involved here.)