For just the avarage electric bike I would expect to pay as little as $400-$500 plus a battery, but it can go up fast with better/faster electronics. A 44.4v 10ah 12s2p turnigy lipo pack would be an example and a 20c 6s 5000mah turnigy is just over $50..
http://www.ebikes.ca/store/store_nc.php
http://ampedbikes.com/kits.html