I was actually just doing some digging around regarding Y-pipes (specifically Termi's Y-pipe) and I found out some interesting info...
I called a Termignoni dealer in Canada (who claimed to have extensive experience with Termi exhaust systems) and he told me that Termignoni makes two different Y-pipes:
-One that comes with the Titanium Competition Full System
-One that you can buy seperately to replace your stocker (stainless steel)
He said that there is a huge difference between the two.
The Y-pipe that comes with the Competition Full System has a much larger diameter at the bottom of the "Y" where it connects to the mid-pipe/headers. But the diameter of where it connects to the exhaust cans are the same diameter as the stainless steel Y-pipe that you can buy seperately. So, the Competition (titanium) Y-pipe will NOT fit stock headers. He also said that the Competition (titanium) Y-pipe is usually NOT sold seperately.
The Termignoni stainless steel Y-pipe that you can buy seperately is designed for the street version of the exhaust (Termignoni Strada Standard Mount Full System), and this Y-pipe has identital diameter specs to that of a OEM R1 Y-pipe.
The stainless steel Y-pipe is only $139.99 (part #446-8000) and can be found here: http://www.exoticsportbike.com/r1_(04-05)1.htm
The Termignoni Y-pipe for $450 that you are talking about may be the Y-pipe from the Titanium Competition Full System.
As far as the Akrapovic Y-pipe...I don't really know much about that pipe, but I did find it (part# 425-2630) for $149.00 at: http://www.exoticsportbike.com/yzf_r1_(04-06)2.htm
Hope that helps.