The cuda pricing, last I knew, will be about as bad or worse. Try having Google support take a look and help with any misconfigurations and account cleanup. By misconfigurations I'm referring to having whitelisted your own domain or users email addresses which would let a lot of spam pass through to your inboxes, and you may also be paying for forwarding for accounts that could be reconfigured as free forwards in Google Apps.
Oh, the old postini service I believe is separate from Google apps. Gmail, Google Apps, and the legacy spam filtering product I believe are all separate even though they're all Google. In that case I still recommend checking for mistakes in your whitelisting rules that might be letting in a lot of spam. Moving from exchange to Office 365 is supposed to be doable too if it's easier to outsource than maintain a Microsoft Exchange server in your office.