Flat rate to FCO from anywhere in the Centro storico is now 40 euros by taxi (the metered legal ones), okay it might be a bit higher at night or early morning, but why stay in Ostia (unless you have business there), when you could stay in Rome proper?
If we hadn't gotten our train tickets earlier (11 or 12 euros each), I would have opted for a taxi straight to the airport. The taxi ride from the Eden to Termini was nearly 10 euros without alot of diversions for traffic.
Argh.