Cat5 cable is cheap. $90 for 300 feet of outdoor cable, I am not sure though if you would need some kind of booster to able to send the signal that far.
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The 300 feet is right, I have seen it work longer, but not always. All you would need is a switch (not a router)every 300 feet. 4 port switches are cheap $40. I have seen people use wifi range expenders, especially if they can put their router in a window of their house, then the range extender in a window of the barn and "jump" the signal over a driveway, etc. Then use standard cable from the extender to the camera.
One suggestion is to put your router in the window, then take your cell phone or ipod on wifi mode to test the range before you buy all the goodies and find out it will not work.
Then of course there are tons of higher dollar sophisticated systems that will send these signals for miles, bridges, antennas, ..... How deep are your pockets?