You're comparing two different markets, as SCION noted. The Fire and Vox aren't aimed at people who want tablets which do everything - they're aimed at eBook readers who want something that'll do their current eBook readers can't do - stream video and music and browse the web.
The Samsung Galaxy Tab 7" Plus is aimed at people who want a fully functional 7" tablet... for the same price you could buy a Galaxy Tab 8.9" at on sale, or an Acer Iconia A500. I don't see Samsung lowering the price anytime soon - they haven't done so yet, and neither have most Android tablet makers. They're still thinking that they can compete with the iPad on equal footing... despite a sub-par experience in Honeycomb. And no ecosystem to support their tablets as it currently exists.