Is it possible?

Is it possible to build a decent e-bike for under $400 US?

YEAH, I wish, but I don't think it is possible with all new components. I have been researching this for some time and this is the least expensive combo that I have come up with.

$ 70.00 Cheap Steel frame bike (Target, Wal-Mart)
$ 255.00 Crystalyte 26 inch 406 Front Wheel (http://www.ebikes.ca/store/)
$ 100.00 36V 20A Start Immediate Brushless Controller (http://www.ebikes.ca/store/)
$ 12.00 Crystalyte Throttle (http://www.ebikes.ca/store/)
$ 75.00 12V 8Ah Battery ($25 each x3 for 36 volts)(http://www.electricscooterparts.com/batteries.html)

$ 512.00 Total

Now this is a bare bones system yet you should get almost 20 mph top speed theoretically (real life top speed may be about 17 - 18 mph) but the down side is the range will not be great with the small batteries.

You may be able to find cheaper batteries on Ebay the thing you don't want to be cheap with is the motor and controller (cause you get what you pay for). I wanted brushless but you could save a little if you get a brushed motor and it would look like this.

$ 70.00 Cheap Steel frame bike (Target, Wal-Mart)
$ 180.00 Crystalyte 26 inch DC Front Wheel
$ 65.00 36V 25A DC Controller
$ 12.00 Crystalyte Throttle
$ 75.00 12 Volt 8 Ah Scooter Battery (approx $25 each x3 for 36 volts)
$ 402.00 Total

So under $400 total could be done If you could pick up a good used bike at a yard sale or on Ebay for $15 to 25 bucks.

Good Luck

I think it could be done. But not with a hub motor. If you use, say, a 500 or so watt motor, a cheap Chinese controller, and lead-acid batteries, I don't think it would cost too much.

Figure:
$100 for a motor (this is a very generous number, you could probably pick one up for $50-60)
$50 for a controller (should be about right)
$100 for batteries (since you want that range with pedalling, it shouldn't cost too much)
$100 for chain wire brackets, switches, etc. (again, probably won't cost you that much)
= $350

Mind, you'll have to figure out a way to mount the motor and work out a drive system, but you asked if it could be done, not if it would be easy.

Here is another option. I know that you said that you wanted to build a bike for under $400.00 but you could buy a complete e-bike for under $400

You may be thinking "What that's crazy talk?" and for the most part I would agree (I was suprised when I found it) but I just saw an E-Zip Trailz Electric Bicycle on Amazon that is selling for $299.99 so even when you add shipping ($65.14) it is still under $400 follow the link

ebike

This is not a top of the line bike by any means, but it is a start and it can always be upgraded later. Here are the specs.

450W of power DC earth magnet motor
Battery: EV rated SLA type Rear rack mounted 24V/10AH pack (2 12V/10AH) Valve regulated, rechargeable
Charge System: UL-listed Currie smart charger LED status display
Controller: Exclusive Currie Electro Drive 24 Volt Fully potted
Top Speed: Up to 18 mph/29 km/h (Rider weight contingent)
Range: Up to 15-22 miles/24-35 km With normal pedaling (Rider weight contingent)
Drive: Patented Currie Electro Drive system Speed-specific gearing
Brakes: Alloy linear pull front and rear Alloy brake inhibit lever
Shimano 7-speed gearing
Wheels: alloy 26" x 2.0" rims
Tires: Currie E-Drive dual use mountain 26" x 1.95"
Net Weight: 76 lbs./35 kg
Rider Age and Max Weight: Young adult to adult
Do not exceed 240 lbs./109 kg

Sure, I did it with a junkyard bike.

WeRelectrified refurbished kits $275 plus shipping ~$25 = $300 complete!(nothing wrong with the 700c I bought almost 2 years ago!) Just keep checking back at the site if they don't have the rim size you want. Just be patient. I can't figure why people think that they have to pay so much for a basic kit? Its just crazy how much companies are charging! We're getting ripped off. The average prices are so high most people should either buy a completely built chinese e bike or just get a high mpg moped! Just my opinions. Good luck.
http://www.werelectrified.com/index.php?cPath=13_14