So many options for runs there is really no set cost per square foot as there are a plethora of options and variables. Looking at a basic rectangle run 10' x 20' w/door. If you have medium to heavier breeds with a low flying ricks, you may want to consider a pasteurization system using elctro netting which is fairly affordable, has diverse applications and can be easy set up and dismantled. The same benefits of free ranging, but more controlled. Recycled wood is always a plus for your usual stick built applications and you can find a lot of pieces just driving around and scanning your local paper, talking to people. The issue with a standard run in my experience is chicken wire is cheap, available and great for growing tomatoes..not great when used for runs as most canines and other predators can chew through it. Hardware cloth is the way to go, but it's considerable more expensive and you get less. Just for some ideas I use clear PVC UV panels on my run roof so they are protected from the elements, but are able to get sun light. About $11 a piece at your local Home Cheapo. If money is a concern, you may want to consider downsizing a few to reduce your square footage needs. Smaller flock will need less room, less money spent feeding and medicating. Also with contained birds, wherever you decide to put them that area will be destroyed with nothing but poop and dirt left over in a matter of a few weeks so keep that in mind for boredom issues.
