I've never been down there for a game myself, but I dug up some info online after I got tickets..... As far as I know, the only major lot down there that is not pre-paid is the Longworth Hall Lot (http://g.co/maps/reugu
). If you have a bus, that's probably your best bet. Its ~$35 per vehicle and opens earlier than the pre-paid lots.
I expect to see lots of Steelers fans. This may be the only sell out at Paul Brown Stadium this year for that reason.