I can help with the control, power supply and input devices but you're going to have to stop by Fantasy Wold for inspration on the motors, props and motor control.
I'm thinking about building a cheap version to see if I can get it to work. My mental parts list for a non GPS driven proof of concept that just hovers would be:
$4 worth of regualr DC motors on PWM channels with cheap Hitachi controllers,
already have Dorkboard w/ 3 to 5 volt boost(this will let me run the acellerometer and board on two AAA batts),
already have accelerometer (Craigslist find a while back),
$15 battery pack/charger for motors/motor controllers,
$10 aluminum sheet from Home Depot to build chassis & arms,
$20ish (4) cheap propellers from fantasy world,
already have screws wire, etc.
This one will probably spend more time in my hand running than in the air but it will start me off with some lessons learned and best practices for under $50. I'd rather crash and burn $50 than $250!