You need to look up the definition of getting ripped off. Because it can basically only mean two things:
1. Not getting what you paid for
2. Being stolen from
When you bought your iMac 30 days ago, you got the model that you paid for. Also, you weren't stolen from. So guess what? You weren't ripped off!
The tech world moves quick, if you can't accept that, then go back to using pen and paper. Apple owes you nothing. Even if there weren't rumors of a pending release, they would still owe you nothing. However, like the other posters said, it was pretty obvious that an update was near. So there's definitely someone to blame for this, but it's not Apple.