Very recently I bought a Thinkpad X200s (12inch screen) from ebay (<£200). I put in 8GB of RAM (£50) (max the chipset will support) and a 256GB SSD (OCZ Octane,

Very recently I bought a Thinkpad X200s (12inch screen) from ebay (<£200). I put in 8GB of RAM (£50) (max the chipset will support) and a 256GB SSD (OCZ Octane, ~£120). Total cost £370, which is just under $600. If you skip the extra ram and get a lower capacity SSD, you can get this under $500. I installed Crunchbang 64bit. I do lots of coding on this machine. My battery lasts about 7 hours. Very lightweight. The Thinkpad keyboard is excellent. I would consider the SSD essential for both speed and battery saving, but you could make it cheaper still without. You could go back another generation and get an X60/61. These are even cheaper. The Thinkpad build quality is excellent. I expect mine to last for at least 5 years.