I picked up an Android 4 7" tablet with 1GB RAM and 16GB flash and 2160P output over HDMI for 105GBP (~165 USD) a couple of weeks ago on Amazon. It came pre-rooted too. (The main reason it's so cheap: 800x480 display that's decidedly lower end than my HTC Desire HD, and obviously the CPU isn't going to set any world records, but all video playback I've thrown at it so far has worked decently).
Also iirc, the Samsung Galaxy S2 tablet versions (i.e. identical to S2 but without the phone) cost around $200 bucks, and that can play any format you throw at it. Plus since it's DLNA capable you could directly stream content off your home server without breaking a sweat. The CPU/GPU are pretty capable being the Cortex A9/Mali 400 combo.
I think it's an integrated feature of at least some of the chipsets incorporating the Mali 400 GPU.
The tablet I got is a NatPC 009S. Also sold as Tabtronic and probably a bunch of other names. It's a nice little thing for it's price. As mentioned, where they've skimped the most is clearly the screen, which doesn't measure up favourably to mid/high end Android phones, and much less to the high end tablets.
Note that I've not tested 2160p output, though, as I don't have anything that can display it, and given the marketing copy on some of these sites who knows, but that's what they claim anyway.
