I tell a lie... they have gone up a bit since I bought mine... well from £2 to £7 lol. But still...http://www.ebuyer.com/search?sort=pricelow&q=2.5%2Bcaddy&limit=10&page=1
and the drives arehttp://www.ebuyer.com/product/172258
The cases are quality bits of kit and serve the purpose well, about £60 for a 500GB external 2.5. Havent seen any of the pre-man drives for that cheap... yet! Give it time though!
DIY of course... but its as easy as ABC... simply plug the drive in to the interface port, slide into the case, secure with two screws and plug into PC using supplied USB cable. They even supply the screwdriver lol.
If it's Windows, it will see it straight away. I get ~20MB/s from mine, which is actually rather fantastic for a USB 2.0 drive (in real terms, not theoretical). It's as good as my Iomega 1TB 3.5 external.