We picked up chain link at the junkyard and got some metal posts out of storage. Our run is about 500 sq. feet (32 ft. long, 16 ft wide) and it cost 50 dollars. We pulled a bunch of rocks out of the garden and got some free bricks at a remodeling project and dumped those around the base to make it hard for animals to dig. It looks pretty good, except for the latch on the gate, which is actually an old dog collar.
I'd check craigslist for "haul away our junk" remodeling projects in your area, and also for pallets, chain link fence, lumber, fence posts... I'd also check out the classifieds in your newspaper, and the junkyard. No need to spend a few hundred dollars at Lowes or Home Depot or whatever your local equivalent is!