It depends on the set-up, I just finished one that adds 3 and a bit kilos including batteries to the weight of the bike, hub freewheels nicely so pedalling is no problem.
If you shop around you can do a front hub for 1.6 kg and an 8 ah LiPo for another 1.1kg so under 3kg is definately doable with out breaking the bank.
A lot of the production ebikes are insanely heavy.