Walmart.com is selling Currie electro drive Ebikes for $350. Surely you'd want to go for something chain driven or hub driven rather than a scrubber drive. The only way doing a bike up like that would be much cheaper is if you had all or most all of the stuff readily at hand. I bought one out of curiosity and so my girlfriend or mother can ride with me and not be left in the dust (even when I'm riding without a motor.) I was really quite impressed by how well it is made and the amount of torque it has for pulling up the hills around here. They come with only one 24v 10ah battery pack rackmounted so dont expect great range stock, but you can add a secondary pack to the rack for like $60. I'd highly suggest checking them out if you're looking for an affordable ebike. Heck, I'll probably ride the thing around a bunch myself when I want to get a bit of exercise while enjoying the sights and SOUNDS without the bother of large hills. The normal top speed of the bike with pedaling the flats was around 18mph. I've seen lots of sites that sell upgraded motors and controllers so I'm sure you could make one fly if you eventually feel the need for Espeed.
This is the one I bought: http://www.walmart.com/catalog/product.do?product_id=8467095
It's the womans version for obvious reasons but the frame and components seem TOUGH! I may even put a revopower wheel on it in the future for the lil lady. If you didn't guess, I wrote the review as well.