I had the same thing happen to my 605 last spring. Fortunately I was still within my warranty. Before getting the repair, I scoured the web, and AVSForum had many people with the same experience. Basically the design on that model led to too much heat buildup, damaging the HDMI board. After mine was repaired I added two 120mm fans to keep mine cool. I'm hoping I get another 3-4 years out of it.

As mine was under warranty, I don't know the exact cost, but I've seen people saying the same as you, $450 or so. At that cost I'd get a new one. If you do stick with Onkyo, from what I have seen the problem was related to that generation. New models are better designed.