Are you planning on building the entire bike from scratch?
What type of drive mechanics do you plan on using?
Are you looking for cargo carrying capabilities?
What are you capable of building?

I'd suggest going scavenger if you are looking to go as cheap as possible.
I'd start looking for a motor that is suitable 1st.
Try looking at flea markets, resale shops & garage sales for something that has an electric motor.
Take a walk around on trash day & just maybe someone will be tossing something useful like an old bike or bench grinder. I'm not suggesting going through peoples trash , but if you spot something that may be useful, grab it up. Craigslist also has listings for free stuff.

See what you can get for cheap or even free. Then when your looking deeply into the heart of your junk pile, inspiration may strike.