How much does it cost to make a bike electronic?

[slipknot01299]

I got a touring bike but I live on dirt roads and dirt roads make me really tired. Im looking to make my bike electric so I wont get tired on the dirt roads then pedal the rest of the way on paved roads. How much does the battery cost and the electric motor cost? Im only gonna use it for 4 miles on my dirt roads then the rest of the way I pedal.