I had a bear get to one of my free-ranging chickens a few weeks ago. Like you said - not really much you can do to stop them. He didn't even try to get into the pen, anyway.
Sounds like you've got a good plan, overall :)
Depending on what the predators are like in your area you're probably not over-engineering. If you have raccoons and nothing else then you'd still need to build a fort. Around $400 sounds about right, unfortunately. Here's a pic of the mine before it was finished.
Six feet long, three wide...