Are you sure you've entered this correctly? Sum(Order Line_Cost) or should this be: Sum ( Order::Line_Cost ) ?
If you meant the second version, calculations that refer to fields in a related table are by defnition unstored and unindexed. Unindexed fields cannot be used as key fields on the child side of a relationship. Thus, what you have set up will not work. Conditional formatting expressions and portal filters (FileMaker 11) do not have this limitation.
If you have FileMaker 11, I'd just use this expression in a portal filter instead of trying to define it in the relationship.
In older versions, you'll probably need to use a script to store the order total in an indexed field. In our invoicing system, we do that by saving the invoice total in an indexed invoice field in a script that the cashier uses to print the invoice. We do that so that we can perform quicker invoice total based searches of our invoices, but it would also work for your portal.
Portal Filter worked great. Just some of the older Filemaker 10 users will not be able to use this correctly. But it will work for now!
THANKS SO MUCH!!