I have a nettop myself, built by myself back before you could get them prebuilt, wound up costing a bit, roughly £150. CurlySteve is right on the build your own being customisable. As an example, you could get a board like this:
Inno3D ION Motherboard with Intel Atom N330 Dual Core DVI VGA HDMI Out Onboard WIFI 6 Channel Audio Mini ITX Motherboard With 80W PSU - Ebuyer
And use whatever case/drive combination you wanted, this CAN cost quite a bit as mini-itx cases are harshly overpriced. There is the prebuilt option where you can get something like this:
Acer Aspire Revo Desktop PC - Desktops at Ebuyer
You're getting pretty good value on something like that, and if it's for playing off of the hard drive or over the network, then it's ideal. If you wanted bluray playback or TV tuning then you'd an external tuner and an external blu-ray drive, which I can't find on ebuyer right now, but you could easily use an internal drive with an enclosure, adapter or with eSata and a power adapter.
But essentially, it's prebuilt to save (some) money but have very limited customisation. For complete customisation, build yourself, considering there are mini-itx boards with sockets on and PCI-E, you can have a full powered PC at tiny sizes.
mini-itx.com used to be the only place to get mini-itx hardware, but ebuyer do it now, generally for better prices, if you're considering building your own, check out this section:
Motherboards Intel - mini-ITX - Ebuyer
When considering cases, check out what you can get cheap elsewhere and use mini-itx.com to get the in-depth specifications, as mini-itx.com do the research into what boards fit in the various cases and things like that.
One final thing, you CAN get mini-itx parts cheap on ebay, it's just not very common. But I've seen a case with fully power supply go for £20, so it might be worth checking out.