I am trying to sumarize the cost of multiple entries in a portal, but that very same portal with the very same information, can appear on multiple records within the same database (they all have a common customer). I am trying to run a billing report for that customer and say, if I have 3 records for that customer, I now have the portal summary 3 times. I only need it summarized once within those 3 records.... how do I have a portal only summarize itself once while showing up over multiple records?

Aggregate functions like count(), max() or sum() can contain a related field so just create a calulation with the function and use the same relationship you use for your portal with the "cost" field.