Summary data from filtered portals is "display only" as far as I have been able to figure out. You'll need to use a different method to compute the same summary total before you can access that data in a script or calculation.
One method is to set up the necessary match fields to match values at the relationship level to reproduce the set of records being totaled in your filtered portal.
Another method, FileMaker 12 only, is to set up a Query with ExecuteSQL that computes the same total.
Both of these approaches can result in a calculation field that computes the same total and then you have a field you can use for exporting this data.
That's what I thought, strange to not have the ability in Filemaker, but I will give ExecuteSQL a try. I have a feeling SQL will be my new goto.
ExecuteSQL is major "game changer" in FileMaker 12. But it also can be frustrating to work with. It's hard to get good feedback when the query fails to work and the results are all crammed into a single field--which can limit it's utility in a number of areas.
But still a major change in capability compared to what Filemaker 11 and earilier can do.