The Adaptive Server optimizer computes the cost of a parallel table partition scan as the largest number of logical and physical I/Os performed by any one worker process in the scan. In other words, the cost of this access method equals the I/O required to read all pages in the largest partition of the table.
For example, if a table with 3 partitions has 200 pages in its first partition, 300 pages in its second, and 500 pages in its last partition, the cost of performing a partition scan on that table is 500 logical and 500 physical I/Os (assuming 2K I/O for the physical I/O). In contrast, the cost of a serial scan of this table is 1000 logical and physical I/Os.
