Anywhere between free and hideously expensive.
Seriously.
If you are on a tight budget for this project, spend some time first scrounging up materials (pallets, plywood, 2x lumber pieces, and anything else relevant you can find.) Then take stock of whatcha got and figure out how best to make it into a coop.
The hardest thing to scrounge, especially in urban or suburban areas, is wire mesh suitable for the run. So you may have to buy that 'full retail'. Cost will depend on what size run you want, and what kind of mesh you use, but you could go price it in your area now so you know what ballpark you're in. I would recommend STOUT 1x1 or 1/2"-hardwarecloth for a small knee-height run; for a walk-in size run, if you are on a budget I would recommend using good stout 2x4 mesh fencing with something smaller-holed added along the bottom. Also remember to figure in the cost of some means of keeping digging predators out -- an apron of wire mesh or scavenged pavers/rubble is often good.
Good luck, have fun,
Pat