http://www.nikonchat.com/remote-nikon-d40/

Can you recommend a good remote shutter for a Nikon D40?
Preferably $ 70
The nikon ml-l3 wireless remote control will cost 15-20 USD. If you mean a cable release similar to those available for higher end Nikons, you're out of luck. The D40 does not have a proper connection for one of them.
Nikon D40