If you don't care about glare from the sun, you can get the LCD for 300.
You can get a motherboard with a built in atom processor for 75 bucks.
Memory will be under 20 bucks.
A cd/dvd drive will be under 30 bucks.
Wifi can be had for under 20 bucks.
Power supply for under 100.
You can probably pull it off for around 600 bucks. If you need tools, you'll really start cranking up the price tag.
This system wouldn't be playing any games, but it would definitely be able to run linux and let you do everything you want to.