I think one day I might get myself a kit I saw advertised on the net, I guess it was something like largoscooters.com.
It has kits as low as 250 dollars. The main attraction is that it uses a hub motor, put in front (so it's a front wheeldrive). The rest of our bike will remain the same, gearing, pedalling, braking....
One minor drawback might only be the extra weight of the hub motor and batteries.