I turned what was once a small garden into our
chicken run a few years back, and did it pretty cheaply. It's held up for about 3 years with no problems so far. I wanted to be overly careful about security against predators, but depending on your situation, you might not have to be as careful. Also, all my materials came from the orange big box hardware store. I also made it expandable, and added a section each year for the last 2 years.
The materials I used are:
4'-height welded wire fence stuff with 2"x4" holes
zip ties, lots of zip ties
4' rebar
some of those green t-post thingies
some 2x4s and 2x3s for the gate (fancy)
staple gun
smallish size PVC and connectors
so basically, it's sort of like the hoophouse someone else described above. The original fence for the garden was buried something like 18" below ground, and I used those green fence stake things and zip ties to give it some support. here's a picture of the old garden circa 2010.
When I started to convert it to a chicken run, I bought some rebar and slammed it into the ground with a mallet right up against the fence stakes, and zip-tied them together. Then I slid pieces of PVC over the ends of the rebar, connected them at the top with a + connector, and added a spine piece of PVC. I forgot if the PVC I bought came in 10 or 12' lengths, but whichever it is, that's what I used, and cut them in half. I think the ribs are just under 4' apart down the length of the run, so I could overlap the top pieces a bit. Here's what it looked like when I started the first few ribs:
and another view:
after the ribs were connected, I started to roll the fencing out over the top. Every seam was connected with zip ties, and the pvc joints were connected with PVC-joining goop.
I was even able to obtain some fence material when the neighbor's barn fell down and he threw everything out from it. with just 2 chickens, you could probably get away with just one of these sections (a rib on each end, and one in the middle. something like 8') we had 6 chickens the first year with that amount of space. of
course, more is better, but you can always add on later.
Here's a picture dump of the design that I just took:
The last picture is my gate. the upright posts are 2x4s nailed together to make cheap 4x4s. the gate door itself is made of 2x3's to be cheaper and lighter. a couple of hinges and other hardware. hope this helps. sorry for the million pictures!