Cheapest way to wirelessly transmit video
You could use a cheap wireless 2.4GHz video extender. These are made to allow distributing the picture from your set-top-box to other TVs in the house.

Usually, they consume and produce composite video, ideal for a cheap camera.

Eg. http://www.amazon.co.uk/Wireless-2-4GHz-Audio-Video-extender/dp/B0047D9XP4