Mozy has both dropped their unlimited plans recently. It looks like for 400GB Mozy is about $38 a month. ($9.99/month for the first 125GB, $2/20GB after that.)
Carbonite still has it, as do other services. I have read a lot of complaints about service levels on Carbonite, though, and I have to wonder if their current price levels are sustainable. It looks like they only allow you to backup your internal drive unless you buy the $99 yearly plan. Still, that's a discount over S3 or Mozy.
I will say I have more faith in S3's servers and redundancy than cheaper options but your mileage may vary. I have been happy with my S3 backups.
You should figure this in combination with local backups. Since you are also using Time Machine, the remote backup is not as essential. After all, you probably won't need the remote backup unless there's a problem with the Time Machine. That means you can probably safely spend less on the remote backup, knowing that if they have some temporary server problems once in a while it's not a problem.