Making bicycle semi-motor powered Is there any way that I could easily and affordably (under $US200) add a motor to my bicycle, so that it would move both by my pedalling and with the help of the motor?
ebay.com, "Electric Bike" brings up dozens of possibilities. "Petrol bicycle" brings up several petrol option. Any of these kits will do the job, but you will get what you pay for.  

Electric kits are expensive, at least twice what you have suggested. 

In the price range you are looking, a petrol motor version might be best. It's tried and old technlogy, is reliable, is cheaper and has a longer range than electric.  Electric does carry the "Green Tax", is new and relatively expensive for good solutions. 

Be aware of local laws as far as limits to motor power and speed go. Most locations limit the power to about 300W and often speeds to below 20km/h or 25km/h (above these the motor is not allowed to provide assistance). 

What research have you done, is there any reason any of these is unsuitable?