Not much will be cheaper than rebar. If you have a chance, make sure
you interlock your corners. I have found this to be the best way to
prevent movement.

If I build one of these, I put the frame or design (if it is just a
wall) and tack it together with some small galvanized nails on the
edges. Once assembled, I take my drill with a spade bit and a 12"
extension (HD or Lowe's, $6) and chuck up the 5/8" bit. Drill every
16 inches if it is on unlevel ground, and only go about 16" into
undisturbed soil.

If the ground is pretty level, then the stakes are only about 24"
apart. In all cases though, you need to make sure your timbers have
some 40d galvanized at the joints if you have them in the field. Also,
no matter what the length, drill though your corner or turn joints and
stake those. I never go more than 16" into the undisturbed soil, and
never use less than a 1/2" bar.

If I were afraid of rust, I would simply spray the bars with car
undercoating before installing. At any rate, I still see one that I
put in many years ago (7-8?), and nothing has failed anywhere on it.
It looks like it has a few years left, and nothing was done to prevent
rust. I don't think rust will be a problem for you, regardless of the
PT you are using. And if it is just plain old landscaping timbers
which are mostly dipped/soaked, not PRESSURE treated, you won't have
problems. When you cut one, you will see what I mean. I don't think
I have ever seen one of those things that the treatment material went
more than 3/4" to 1" into the wood.

I line mine completely with plastic like visqueen, rolling the plastic
all the way up to the edge to be but off later for appearance sake.
This will help keep unwanted plants from sneaking in as a root or
runner.

Robert