3/4" ply would be a smart choice -- the cost savings of stepping down to 1/2" isn't worth it. As far as cost relative to pine, unless you're working in oak-faced ply, you should end up saving some dough by using ply. Figure on getting about 100 linear feet of material from a sheet of ply (12 strips, 8 feet each). I did this with nice Arauco-faced ply at around $30 a sheet, and figure I paid somewhat less than for pine (would have been around $50.) With pine, you'll also have to spend a good amount of time making sure you're getting good stock -- no absurd warp, twist, bow, checking, knotting, etc etc etc.
As far as working with the ply, any half-decent circular saw or skilsaw will turn a 4x8 into a pile of furring strips in just a few minutes.
I used box-frame construction, rather than L-girder, because my layout is pancake-flat, but one thing that I tried and I've been extremely happy with was the use of pocket-hole joinery. This gives a nice clean look, with a super-strong joint, and no splitting at all. Drilling the holes is an extra step, but then, so is drilling a pilot hole in any other type of contruction.
My blog is horribly outdated (I think I only did three or four posts) but you'll find one or two half-decent photos there, if you're interested.
Peter
