If I may be so bold, my friend, that day has already arrived. The price however, is that of Lithium battery.
I can tell you that I am able to go more than 50 miles on mine, and depending on how much you want to pedal, and how fast you need to go, you can really do as much as you want (or your wallet can handle).
I have been having good success with this beauty:
Here's my build thread:
http://endless-sphere.com/forums/viewtopic.php?f=3&t=21173&start=60
Total added weight to bike, 5 - 10 lbs depending on how much battery you need, I use about 15lbs total (drive and battery), but I also went 53 miles the other day and only used 2/3's of my battery pack.
I am setting up a demo bike with this drive at the local bike shop to try and promote the product (I can't do a regular 9 to 5 because of my condition) so hopefully building electrics will work for me.
Don't just take my word for it though, there are others building them too.
If you do decide to try this, just make sure that you follow the recommendations on motor and and battery voltage. After building a few hub motors I thought I "knew" what I was doing using way too much voltage and burned up a motor and controller.
I was spinning my rear tire at 62 MPH no load speed!!
And I was fine (not using full throttle because it was SCARY fast acceleration!!) on the flat, but after hitting about 36MPH at warp speed I backed off my dial-throttle (servo tester, no return spring hehe) and then thought I was doing ok.
Then I went up a local 8% hill was flying past some poor sod slugging up the hill at 20MPH, when I let out the magic smoke!
Now that I have the correct motor, voltage and controller, I can do the same hill at 16MPH with moderate pedaling and a bike that weights only about that of a walmart bike (41 lbs).
Here is the youtube vid of the TV show "the new inventors" with the maker of the drive:
YouTube - Kepler_Drive_on_New_Inventers.wmv
If you have any more questions, I can send you some other links of other guys building their own with this kit.
Dayn