The optimizer computes the cost of a hash-based table scan as the total number of logical and physical I/Os required to scan the table.
For an allpages-locked table, the physical I/O cost is approximately the same as for a serial table scan. The logical cost is the number of pages to be read multiplied by the number of worker processes. The cost per worker process is one logical I/O for each page in the table, and approximately 1/N physical I/Os, with N being the number of worker processes.
For a data-only-locked table, this is approximately the same cost applied to a serial table scan, with the physical and logical I/O divided evenly between the worker processes.