If it is for driving, think about a mowed arena -rather than a sanded one. If you live in the right climate, it can work out well. It will drain very fast and if seeded with a tough type of grass, can hold up very well.
There are lots of ways to create a barrier/lines of arena without a fence. Also, by having a movable barrier -it allows for grading or mowing more easily.
If it is mowed, just the difference between the cut arena and the long grass outside of the arena can be enough of a difference for a ring. This can be very successful.
Even cones can work to define a space (and allow for easy mowing or harrowing of arena).
Other ideas for fencing:
What about white polytape (electric) -strung on plastic step in posts (but no eletricity),
The construction plastic flags that just stick into the ground.
Landscaping posts just placed on the ground with a 20 feet gap between each.
Straw bales are cheap and spaced about 20 feet apart around an arena can work. At fifty feet apart, you would need about 20 x $3. = $60. They last about a year.
Thanks for the suggestions.
It would definitely be a grass arena.
As for the fencing my main concern is that in the event of an accident I would have a loose pony. God willing with the right preparations and instruction this will be a mute point, but with the highway so near I don't want to risk it. What is considered a safe enclosure for driving as far as fence type and height ? Can you think of any safety issues that someone new to driving might overlook in designing a arena ?