I don't want to be discouraging, but the costs of the port at Ostia are going to be nearly impossible to pin down, since 1) its storied founding was in 610 BC by a king many folks these days believe was a diety personification rather than a real person, and 2) finding subsequent maintenance or rebuilding records (e.g. after it was razed by good old Marius) is nigh on impossible as far as I know.

Your best bet is to hit a library and look for anything you can find on Roman maritime trade. However, there are few original sources that deal with this area.

Good luck!