The guys at Dragonflypower.com
have the cheapest automatic chicken coop door design I could find
during my search.
It's use of a basic alarm clock keeps the cost low, but I've
got some serious doubts on how solid of a solution this is going to be
for the long haul.
The complete design is being given away at their website in
what looks like an effort to promote their build your own wind
generator plans, which are 10 dollars.
Edited to add:
After years of research, Mark eventually settled on this automatic chicken door.
You can see
a summary of the best
chicken door alternatives and why he chose this version here.
If you're planning on
automating your coop, don't forget to pick up one of our chicken waterers. They never spill or
fill with poop, and if done right, can only need filling every few days
or weeks!
We do feed our chickens, about 2/3 of a cup of laying pellets per chicken per day. Chickens can get about 10% of their food from pasture, although Harvey Ussery is doing a lot of experiments on growing his own chicken feeds and finding other ways to drastically cut feed costs. (There's a lot of information on his website at http://www.themodernhomestead.us/ and he's also had some great articles in Backyard Poultry magazine.)
The amount of natural food the chicken tractor lets your chickens eat depends on how often you move it. I nearly always move my tractor every day, which gives the chickens fresh weeds and bugs. I highly recommend you build a tractor --- we'd never go back to a coop!