Anything you buy professionally made will cost a ton. Look at stealth electric bikes if you want something really cool, but they are $6000+. Building yourself you can get it done for $1000-1500 depending on the frame. Turnigy lipos are something like $42 for a 5ah 6s pack in the US warehouse with $.01 shipping during a swarm sale, and 12 fet "lyen" programmable controllers are under $150.
If you want to toy around with setups, there is some GREAT prediction softwre -
http://endless-sphere.com/forums/vie...php?f=2&t=6892
The bike you linked to looks pretty good. I actual bike probably costs $1000+ and after the components and labor the price is actually pretty good. If you have the money and you don't have time, I would probably go for it unless you want full suspension.