For years I rode in my pasture, no access to an arena. 2 years ago, my dad decided he needed some dozer work done, and had a spot scraped level for an arena. We used landscaping timbers (the ones that are flat on 2 sides) and regular wooden fenceposts in the corners (corners cemented in for stability, but that's kind of above and beyond the call of duty, esp. for a makeshift arena). We bought this stuff that looks a lot like hot fence tape, but doesn't have wire through it, just white nylon fiber, about 4 inches wide. I think, not including dozer work, we have around $350 invested in the arena materials, but we did all of the work ourselves, digging holes for posts, measuring and affixing the tape, etc. We want to eventually add something to improve the footing, but the arena itself turned out really nice.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown